EMMANUEL DUNAND/AFP/Getty Images
A family pays tribute to the victims of an elementary school shooting in Newtown, Connecticut, on December 15, 2012.
Authorities are cautioning that "weeks worth of work" lie ahead before they can piece together a full picture of Friday's mass shooting at a Newtown, Conn., elementary school, that killed 26 young students, teachers and staff.
Speaking at a news conference Sunday, Connecticut State Police Lt. J. Paul Vance said forensic analysis and interviews of witnesses would take considerable time.
"I am confident that we will put every single resource into this investigation ... and our goal is to answer every single question," he told reporters.
Vance said authorities had spoken to "many, many witnesses, but there are many more." He said the interviews might extend to children who survived the attack. He said there were an "immense number of witnesses."
"There is weeks worth of work that needs to be done to complete this [investigation]," he said.
Vance also warned members of the media and public that "misinformation" was being disseminated via social media sites.
He said that such unofficial sources "cannot be confirmed and in many cases [are] inaccurate." Vance elaborated that "threatening and inaccurate" information was coming from "people posing as other people" on social media.
Vance confirmed that the school had been "forcibly entered." He also said that four weapons had been linked to the crime. Earlier, authorities had said three were found at the school.
NPR's Carrie Johnson says a law-enforcement source has told NPR that the fourth weapon was found at shooter Adam Lanza's home, where it was used to Nancy Lanza, the assailant's mother. The official tells NPR that all four weapons used in the crime were purchased legally by Ms. Lanza.
Separately, Ginger Colbrun, a spokeswoman for the Bureau of Alcohol Tobacco and Firearms, tells NPR that ATF agents have fanned out across the greater Newtown area to visit gun stores and shooting ranges to possibly learn more about the gunman's activities in the weeks, months and even years before the attack.
Adam Lanza, 20, entered Sandy Hook Elementary armed with two handguns and a semi-automatic rifle Friday morning and gunned down 20 children and six adults, including the school's principal, before killing himself.
Update at 10:35 a.m. ET: Connecticut governor offers details about Lanza's death:
Conn. Gov. Dan Malloy tells ABC's This Week that authorities believe Lanza shot himself as first responders closed in.
"We surmise that ... he heard responders coming and apparently at that, decided to take his own life," Malloy said.)
NPR's Susannah George, reporting from Newtown, says, "there is a constant stream of people coming and going from the makeshift memorial at the entrance to Sandy Hook Elementary School. Many people have come from out of town - some as far as an hour away - to lay flowers and light candles."
Twelve girls and eight boys were killed. Sixteen of them were just 6 years old and four were only 7. School officials say Principal Dawn Hochsprung, 47, and school psychologist Mary Sherlach, 56, were shot and killed as they tried to stop Lanza. Teacher Anne Marie Murphy, 52, was killed as she shielded her students, authorities say. Teachers Victoria Soto, 27, Rachel Davino, 29, and substitute teacher Lauren Garbrielle Rousseau, 30, were also among the dead. Nancy Lanza, the first of the six adult victims, was a former stockbroker, who divorced the shooter's father in 2008.
The Harford Courant quoted former classmates of Adam Lanza at Newton High School describing him as a "skinny, shaggy-haired boy 'who never really talked at all'" who was often seen carrying his laptop. The paper writes:
There was a common refrain among acquaintances of Adam Lanza: I knew of him but I didn't know him.
Lanza kept to himself. Over several bloody minutes Friday morning, armed with a rifle, Lanza emerged from his shell long enough to destroy the lives of 20 first-graders and six adults at Sandy Hook Elementary School. He'd gone to the school as a youth, a former classmate said.
Lanza, who lived with his mother in a colonial home in an upscale neighborhood of Newtown, was estranged from his brother, Ryan, and hadn't spoken in four years with his father, Peter Lanza, who is an executive at General Electric. The elder Lanza released a statement on Saturday expressing shock and sadness over the killings.
Ryan Lanza, who was originally misidentified as the shooter, lives in Hoboken, N.J.
The assailant's aunt, Marsha Lanza, spoke with reporters from the doorstep of her home in suburban Chicago on Saturday. She described her nephew as "a very bright boy," who was "different" and quiet.
" ... nice kid good kid, I mean he was definitely challenging family in that house. Every family has one, I have one, they have one, but never in trouble with the law, never in trouble with anything," she said.
NPR's Brian Naylor reports that law enforcement officials would not confirm that Lanza had Asperger's Syndrome, a form of autism that is characterized by social awkwardness and other difficulties in relating to others.
The aunt said Lanza's mother "had issues with school she eventually wound up home-schooling him and she battled with the school district. In what capacity, I'm not certain if it was behavior [or] if it was learning disabilities."
Naylor reports that authorities believe the guns used in the assault were all legally obtained by Nancy Lanza.
"A number of reports have quoted family friends saying she and her son had gone to area shooting ranges together," he says.
The New York Times quotes Dan Holmes, an acquaintance of Ms. Lanza, as saying she had several different guns, "I don't know how many," he told the newspaper, adding that "She would go target shooting with her kids."
While police investigators have so far offered no motive for the deadly assault, Gov. Malloy suggested Saturday that a truly complete picture might never be known.
"When tragedies like this take place, people often look for answers, an explanation how this could have happened. The sad truth is there are no answers. No good ones, anyway," Malloy said.
Later Sunday, Obama was to meet privately with victims' families at Newtown. According to The Associated Press, the president would also meet with emergency personnel who responding to the shooting. Later, he will speak with a vigil at Newtown High School.
The visit will be Obama's fourth as president to a community that experienced a mass shooting.
Separately, The New York Times reports that following the shooting of Rep. Gabrielle Giffords in Tuscon, Ariz., the Department of Justice drew up guidelines for an expanded firearms background-check system aimed at keeping guns out of the hands of mental ill people and criminals, but that the plans were later shelved "as the election campaign heated up and as Congress conducted a politically charged investigation into the Operation Fast and Furious gun trafficking case."
Update at 10:25 a.m. ET: Feinstein pledges to revive proposed ban on assault weapons:
Sen. Dianne Feinstein (D-Calif.), speaking on NBC's Meet the Press, says she will revive a plan to ban new assault weapons.
Meanwhile, Connecticut independent Sen. Joe Lieberman tells Fox News Sundaythat a national commission could be formed to look into gun laws, the mental health system and the possible role that violent video games and movies might play in mass shootings.