From Susan Candiotti and Greg Botelho
NEWTOWN, Connecticut (CNN) -- Relatives of those killed in an elementary school massacre met Sunday night with President Barack Obama, trying to make sense of bloodshed that has rocked not only the Connecticut community of Newtown, but the nation and the world.
Twenty-six people -- 20 of them children, ages 6 and 7 -- were killed Friday morning at Sandy Hook Elementary, before the shooter fatally shot himself. Hours later, a tearful Obama mourned the "beautiful little kids" slain and regretted that, "as a country, we have been through this too many times."
The president offered his condolences personally early Sunday evening, as he met with the victims' families at Newtown High School. He later joined a nighttime memorial service in the same building.
"We needed this," said the Rev. Matt Crebbin from Newtown Congregational Church, the event's first speaker. "We needed to be together."
Adam Lanza, 20, has been positively identified as the gunman, Connecticut State Police Lt. J. Paul Vance said Sunday. The police spokesman also revealed that the shooter's mother, Nancy Lanza, is the woman found dead from "multiple gunshot wounds" in the Newtown, Connecticut, home she lived in with her son.
While more details have emerged about Friday's massacre in this quiet New England town, authorities still don't have an explanation as to why it happened.
"We will and we are searching diligently and nonstop to attempt to answer that," Vance said.
Details emerge about school massacre
The first authorities heard of the shooting at Newtown's Sandy Hook Elementary School came in a call around 9:30 a.m. Friday.
It was then Lanza got into the building -- not by being buzzed in, though a security system recently implemented by Principal Dawn Hochsprung, but by firing. Hochsprung was among the six adults killed.
Lanza used a Bushmaster AR-15 "assault-type rifle" to gun down children in two classrooms, according to Vance, then killed himself with a handgun. "Multiple magazines," each of which contained 30 bullets, were fired from the Bushmaster weapon in the school, he added.
"We surmise that it was during the second classroom episode that he heard responders coming and apparently at that (point) decided to take his own life," said Connecticut Gov. Dannel Malloy on ABC's "This Week."
Authorities still have witnesses they plan to interview, Vance said, and they're analyzing evidence. The investigation could take weeks, he said.
Vance said authorities also are looking into threats and inaccurate information about the shooting on social media and elsewhere. He didn't give specifics, beyond referencing a threat to St. Rose of Lima Roman Catholic Church in Newtown that prompted its evacuation.
"Anyone who harasses, threatens or intimidates or interferes with the investigation -- utilizing any social media of any type relative to this horrific crime -- will be fully investing and fully prosecuted to the extent of the law," the police spokesman said.
Medical examiner: 'The worst I have seen'
All the victims died after being shot multiple times, said H. Wayne Carver II, Connecticut's chief medical examiner. Their deaths -- as well as that of Nancy Lanza -- are classified as homicides.
"This probably is the worst I have seen or the worst that I know of any of my colleagues having seen," said Carver, who personally performed autopsies on seven victims.
As to Lanza, the gunman, the few relatives and acquaintances who have talked publicly are at a loss to explain how this could have happened. He had no known criminal record. He visited a gun range, as did his mother, said ATF spokeswoman Ginger Colbrun.
An aunt and a former classmate described him as very intelligent and quiet. His father, Peter Lanza, released a statement Saturday expressing condolences to victims' families and saying his family is in a state of disbelief.
As a teenager, Lanza often sat alone in the back of the school bus, his former bus driver said.
"He didn't sit with the other kids and didn't seem to have any friends," said Marsha Moskowitz, 52, who said she drove Lanza to school for three years starting when he was age 13.
Lanza was one of the older kids on the bus and did little to interact with the others, she told CNN. "He was quiet, a very shy and reserved kid."
Senator says she'll introduce assault weapons ban
The deadly shooting that rocked Newtown and reverberated around the world also reignited the debate about gun laws.
Democratic Sen. Dianne Feinstein of California said Sunday that she will introduce a bill next month to prohibit assault weapons.
"It will ban the sale, the transfer, the importation, and the possession. Not retroactively, but prospectively. It will ban the same for big clips, drums or strips of more than 10 bullets," Feinstein said on NBC's "Meet the Press."
Adam Lanza was found dead next to three guns -- the semiautomatic .223-caliber rifle made by Bushmaster and two handguns made by Glock and Sig Sauer. A fourth weapon, a shotgun, was found in Lanza's car, according to Vance. Additional weapons were found in his house, a law enforcement source with knowledge of the investigation said.
The weapons belonged to the gunman's mother, Nancy Lanza, who 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.
Besides the three weapons found at the school, the shooter also had access to at least three more guns, a law enforcement source said.
On Sunday, Connecticut's governor said federal officials need to do more to regulate guns.
"These are assault weapons. You don't hunt deer with these things. ... One can only hope that we can find a way to limit these weapons that really have one purpose," Malloy said.
Devastation in Newtown
The massacre in Newtown is the second-deadliest school shooting in U.S. history, behind the 2007 Virginia Tech shooting spree that left 32 dead.
"Stuff like this does not happen in Newtown," said Renee Burn, a teacher at another school in the town, which is roughly 75 miles northeast of New York.
Until Friday, only one homicide in 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 shooting wounded the entire community, said Sen. Richard Blumenthal, D-Connecticut, who has met with victims' families and watched devastated parents learn their children had been slain last week.
"I don't think I will ever forget the cries of grief and pain that I saw at the firehouse on that day," he said, "as a parent, as a person, just the unspeakable sadness that pervades this town still, and will go on for quite some time."
CNN's Susan Candiotti reported from Newtown, Connecticut, and Greg Botelho reported from Atlanta. CNN's David Ariosto, Candy Crowley, Dana Ford, Catherine E. Shoichet, Ashleigh Banfield, Joe Johns, Terry Frieden, Michael Martinez and Chuck Johnston contributed to this report.
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