Police: Gunman forced his way in

NEW: Source: Lanza tried to buy a gun earlier in the week NEW: Source says Lanza had an altercation with four adults at the school on Thursday NEW: A first-grade teacher and substitute teacher are among the dead Detectives find "very good evidence" at the school and suspect's mother's house

Watch CNN's LIVE TV coverage of the Connecticut elementary school shooting as the story continues to unfold.

Newtown, Connecticut (CNN) -- Major crime investigators from the Connecticut state police on Saturday were combing "every crack and crevice" of Sandy Hook Elementary School in Newtown, a day after a gunman shot dead 20 students and six adults before apparently killing himself.

They said they're finding some "very good evidence" in their search of the school, and at the home of the man identified by authorities as the shooter -- 20-year-old Adam Lanza. Lanza's mother was killed at that house before the school rampage began, authorities said.

"The detectives will certainly analyze everything and put a complete picture together of the evidence that they did obtain, and we're hopeful -- we're hopeful -- that it will paint a complete picture as to how and why this entire unfortunate incidence occurred," said Lt. J. Paul Vance, a spokesman for the Connecticut State Police.

Vance said police are already talking to the one wounded adult at the school, a woman who has not been named.

"She has been treated and she'll be instrumental in this investigation, as I'm sure you can understand," Vance said.

Thursday, the day before the shootings, Lanza was involved in some kind of altercation at Sandy Hook Elementary, a law enforcement source told CNN.

The disagreement was between Lanza and four adults, three of whom were killed Friday, the source said. It's not clear whether the altercation happened inside or outside the school, but it had something to do with Lanza trying to enter the school, the source said.

Two days before the argument, Lanza tried unsuccessfully to buy a gun at Dick's Sporting Goods in nearby Danbury on Tuesday, according to a law enforcement source with knowledge of the investigation. The source said store employees have been interviewed and have searched the store's surveillance cameras for evidence that Lanza was there.

Lanza was found dead next to three guns, a semi-automatic .223-caliber Bushmaster rifle and two pistols made by Glock and Sig Sauer, a law enforcement source told CNN. All belonged to his mother.

Nancy Lanza was a gun collector and recently showed off a newly bought rifle to fellow Newtown resident Dan Holmes, who owns a landscaping business in the town.

"She told me she'd go target shooting with her boys pretty often," Holmes told CNN.

Lanza also had access to at least three more guns, a second law enforcement source said. Investigators recovered a .45-caliber Henry Repeating Rifle, a .22-caliber Marlin Rifle and a .30-caliber Enfield Rifle, though it's unclear where they were found, the source said.

Authorities have identified all of those killed in Friday's tragedy, but they are not releasing a formal list of names and birthdates until the state medical examiner has completed his work, Vance said Saturday morning.

.cnnArticleGalleryNav{border:1px solid #000;cursor:pointer;float:left;height:25px;text-align:center;width:25px} .cnnArticleGalleryNavOn{background-color:#C03;border:1px solid #000;float:left;height:25px;text-align:center;width:20px} .cnnArticleGalleryNavDisabled{background-color:#222;border:1px solid #000;color:#666;float:left;height:25px;text-align:center;width:25px} .cnnArticleExpandableTarget{background-color:#000;display:none;position:absolute} .cnnArticlePhotoContainer{height:122px;width:214px} .cnnArticleBoxImage{cursor:pointer;height:122px;padding-top:0;width:214px} .cnnArticleGalleryCaptionControl{background-color:#000;color:#FFF} .cnnArticleGalleryCaptionControlText{cursor:pointer;float:right;font-size:10px;padding:3px 10px 3px 3px} .cnnArticleGalleryPhotoContainer cite{background:none repeat scroll 0 0 #000;bottom:48px;color:#FFF;height:auto;left:420px;opacity:.7;position:absolute;width:200px;padding:10px} .cnnArticleGalleryClose{background-color:#fff;display:block;text-align:right} .cnnArticleGalleryCloseButton{cursor:pointer} .cnnArticleGalleryNavPrevNext span{background-color:#444;color:#CCC;cursor:pointer;float:left;height:23px;text-align:center;width:26px;padding:4px 0 0} .cnnArticleGalleryNavPrevNextDisabled span{background-color:#444;color:#666;float:left;height:23px;text-align:center;width:25px;padding:4px 0 0} .cnnVerticalGalleryPhoto{padding-right:68px;width:270px;margin:0 auto} .cnnGalleryContainer{float:left;clear:left;margin:0 0 20px;padding:0 0 0 10px} People attend a prayer service in Newtown to reflect on December 15. A mother hugs her children after paying tribute to the victims in Newtown on Saturday, December 15. Newtown High School student Trevor Lopez stands outside of a church where residents have come to pray and reflect on December 15. Andrea Jaeger places flowers and a candle at a makeshift memorial outside a firehouse near Sandy Hook Elementary School on December 15. The U.S. flag flies at half-staff above the White House on December 15. Ken Kowalsky and his daughter Rebecca, 13, embrace while standing at the end of the road leading to Sandy Hook Elementary School on December 15. A woman puts a flower near crosses planted by Rio de Paz (Rio of Peace), in memory of the victims of the Sandy Hook Elementary school shooting on Copacabana beach in Rio de Janeiro on December 15. A woman sits during a service at the Asylum Hill Congregational Church in Hartford, Connecticut on Friday, December 14. Fans at the NBA game between the Utah Jazz and the Phoenix Suns participate in a moment of silence for the victims of the Newtown shooting on December 14 in Phoenix, Arizona. An overflow crowd listens to a church service held at the St. Rose of Lima Catholic Church in Newtown on December 14. People gather for a prayer vigil at St. Rose Church on December 14. People gather for a vigil outside the White House in Washington following the Connecticut elementary school shooting on December 14. Candles burn as people gather for a vigil outside the White House on Friday. Hartford, Connecticut, Mayor Padro Segarra speaks emotionally about the students and teachers who died earlier in the day at Sandy Hook Elementary School in nearby Newtown at a candlelight vigil at Bushnell Park in Hartford on December 14. Cynthia Alvarez is comforted by her mother, Lilia, as people gather for a prayer vigil at St. Rose of Lima Roman Catholic Church in Newtown. People gather for a prayer vigil at St. Rose Church in Newtown on December 14. People gather for a prayer vigil at St Rose Church in Newtown on December 14. People gather inside the St. Rose Church to remember the shooting victims on December 14. A woman bows her head during a vigil for the shooting victims on December 14. People gather in the St. Rose Church during a service on December 14. Women comfort each other during the vigil at St. Rose Church on December 14. A woman looks on during the vigial on December 14. People hug outside of the Newtown United Methodist Church on December 14, in Newtown, Connecticut, near the site of the shootings at the Sandy Hook Elementary School. A flag at the U.S. Capitol flies at half-staff after President Barack Obama ordered the action while speaking from the White House. Obama called for "meaningful action" in the wake of the school shooting. Julie Henson of San Francisco joins other people outside the White House to participate in a candlelight vigil to remember the victims at the Sandy Hook Elementary School shooting in Newtown, Connecticut on December 14, 2012. According to reports, the gunman killed 20 children and six adults. U.S. President Barack Obama wipes tears as he makes a statement in response to the elementary school shooting in Connecticut on December 14. Washington resident Rachel Perrone, left, and her 5-year-old son, Joe, center, join others outside the White House in a candlelight vigil on December 14. Faisal Ali, right, of Colorado Springs, Colorado, joins the vigil outside the White House on December 14. People gather outside the White House to participate in a candlelight vigil. Supporters of gun control hold a candlelight vigil for victims of the Sandy Hook Elementary School shooting outside the White House on Decmber 14. Newtown school shooting: World reacts Newtown school shooting: World reacts Newtown school shooting: World reacts Newtown school shooting: World reacts Newtown school shooting: World reacts Newtown school shooting: World reacts Newtown school shooting: World reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts Newtown school shooting: world reacts HIDE CAPTION << < 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 > >> Newtown school murders: World reacts

Among the dead are Dawn Hochsprung, the school's beloved principal, and school psychologist Mary Sherlach.

Based on CNN's contacts with friends and family members, CNN was able to identify two other adults killed at the school: Vicki Soto, a first-grade teacher, and substitute teacher Lauren Rousseau.

Vance said Saturday that Lanza forced his way into the school, though he wouldn't say how or whether Lanza used weapons to do it.

Authorities said it's also not clear whether Lanza entered before or after 9:30 a.m., the time each day when the school would lock its doors as part of a security system introduced this year. Authorities say the first emergency call about the shooting came in at "approximately" 9:30 a.m. Friday.

Within minutes, 26 people had been killed with chilling efficiency, leaving only the one wounded survivor, according to Vance.

"Stuff like this does not happen in Newtown," said Renee Burn, a teacher at another school in the town, which is roughly 60 miles northeast of New York City.

Until Friday, only one homicide in the past 10 years had been reported in the upscale community of expansive homes surrounded by woods, where many residents commute to jobs in Manhattan and the nearby Connecticut cities of Stamford and Hartford.

The number of young victims, between the ages of 5 and 10, sent shockwaves across the nation.

With the death toll at 26, the massacre in Newtown is the second-deadliest school shooting in U.S. history, behind the 2007 Virginia Tech mass shooting that left 32 dead.

What really makes schools safer?

Flags were lowered to half-staff in a number of states, and vigils were held at houses of worship and at schools amid a national outpouring of grief.

Two law enforcement sources said Adam Lanza lived with his mother. Contrary to early reports, they said, Nancy Lanza was not a teacher at Sandy Hook Elementary.

Investigators believe Lanza killed his mother and then took her guns and made his way to the elementary school wearing black fatigues and a military vest, according to a law enforcement official.

At about 9:30 a.m., as announcements were read over the loudspeaker to the nearly 700 students, the first shots rang out.

Students described being ushered into bathrooms and closets by teachers after hearing the first shots.

It sounded like "pops, gunshots," Janet Vollmer, a kindergarten teacher, said.

Vollmer locked her classroom doors, covered the windows and moved her 19 pupils toward the back of the room.

"We're going over in a safe area," she told the 5-year-olds. Then, she opened a book and started to read.

Outside Vollmer's classroom, a gunman was moving through the hallway of the one-story building.

In the first few minutes, the gunman is believed to have shot the principal, Hochsprung, and the school's psychologist, Sherlach.

One parent who was at the school in a meeting with Hochsprung, Sherlach and the vice principal said she heard a "pop, pop, pop." All three left the room and went into the hall to see what was happening. The parent ducked under the table and called 911.

"I cowered," she told CNN. The gunman "must have shot a hundred rounds."

At the police station, dispatchers began to take calls from inside the school.

School shooting: Shattering the sense of safety

"Sandy Hook school. Caller is indicating she thinks someone is shooting in the building," the dispatcher told fire and medical personnel, according to 911 tapes.

Then, another caller reported gunshots. And then another.

"Units responding to Sandy Hook School at this time; the shooting appears to have stopped. The school is in lockdown," the dispatcher said.

The dispatcher warned police and medical personnel that callers were reporting "multiple weapons, including one rifle and a shotgun."

Then, a police officer or firefighter called for "backup, ambulances, and they said call for everything."

The dispatcher, according to the 911 tapes, asked how many ambulances were needed.

"They don't know. They're not giving us a number," the officer or firefighter said.

Inside a classroom, Vollmer was still reading to the children when police officers banged on the locked door.

The kindergartners were told to line up and cover their eyes as they were led by police past bodies, presumably of their fellow schoolmates, Vollmer said.

As reports of the shooting made their way around town, frantic parents descended on a nearby firehouse where the children had been taken.

"Why? Why?" one woman wailed as she walked up a wooded roadway leading from the school.

Inside the firehouse, Vollmer's kindergartners were beginning to understand something terrible had happened.

"They saw other people upset," Vollmer said. "We just held them close until their parents came."

10 ways to put brakes on mass shootings in schools

By nightfall, the firehouse became a gathering point for parents and family members whose loved ones would never walk out of the school.

Authorities, meanwhile, in Hoboken, New Jersey, were questioning Ryan Lanza, the suspected gunman's older brother, law enforcement sources said, though they did not label him a suspect. Lanza's father, Peter, who lives in Connecticut, was similarly questioned, one of the law enforcement officials said.

People are sharing their concern and sadness over the Newtown school shooting. What are your thoughts? Share them with CNN iReport.