By Susan Candiotti and Chelsea J. Carter, CNN
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.
Authorities have now 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, said Lt. J. Paul Vance, a state police spokesman.
The investigators are looking inside and outside the school building and at every car in the parking lot, in what Vance called a "long and painstaking process" that will not be completed for at least another day or two.
Law enforcement officials identified the shooter as 20-year-old Adam Lanza, who grew up in the tight-knit community of 27,000. Police say Lanza killed his mother, Nancy, at her Newtown home before going to the school, where he primarily targeted two classrooms of kindergartners and first-graders.
Lanza's motive was unclear Saturday, though Vance said detectives are collecting some "very good evidence" at both the school and his mother's house. He would not say what the evidence includes.
"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," Vance told a packed crowd of journalists at a Saturday morning news conference.
Lanza forced his way into the school, Vance revealed Saturday, though he wouldn't say how or whether Lanza used weapons to do it.
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, Lanza killed 26 people with chilling efficiency, leaving only one wounded survivor, according to Vance. Among the adults killed were Dawn Hochsprung, the school's beloved principal, and school psychologist Mary Sherlach.
"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.
Authorities found three guns next to Lanza's body in one of the classrooms, a law enforcement source told CNN. All three -- a semi-automatic .223-caliber Bushmaster rifle and two pistols made by Glock and Sig Sauer -- were owned by Lanza's mother, the source said.
Investigators know which one Lanza used to kill himself but are not yet revealing that information, the source said.
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.
One of the law enforcement sources said they have information that Lanza tried to buy a gun in the area this past Tuesday. The Bureau of Alcohol Tobacco and Firearms said it is contacting shooting ranges and gun stores in the area to try to establish whether Lanza sought to purchase guns or practice using them.
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.
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.
"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."
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.
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CNN's Susan Candiotti and David Ariosto reported from Newtown, Connecticut; Chelsea J. Carter from Atlanta. CNN's Sarah Aarthun, Meredith Artley, John King, Ashleigh Banfield, Joe Johns, Terry Frieden, Michael Martinez, Dana Ford and Greg Botelho contributed to this report.