Police are investigating reports that Connecticut gunman Adam Lanza had an altercation at the Sandy Hook Elementary School in the days before he carried out a massacre the school's students and staff, sources told ABC News.
Authorities indicated today that they have "some very good evidence" about the motive behind Lanza's shooting spree at the school in Newtown, Conn., and said that the sole person to survive being shot by Lanza will be "instrumental" in the probe.
The grim task of identifying all of Lanza's 27 victims, which included 20 children, was completed today. Families, who already feared the worst, were informed that their loved ones were dead early today.
All of the bodies have now been removed from the school and medical examiners are expected to provide a full list of victims later today.
With the tally of Lanza's carnage complete, authorities and the grieving people of Newtown, Conn., are left to wonder why he turned the elementary school in this quaint New England town into a slaughter house.
Connecticut State Police Lt. Paul Vance, who had compared the investigation to "peeling back the layers of an onion," said the investigation "did produce some very good evidence" about motive, but he would not go into further detail.
He indicated the evidence came from the shooting scene at the school as well as at the home where Lanza's mother, Nancy, was slain.
Also key will be the lone person shot by Lanza who wasn't killed. The female teacher has not been publicly identified.
"She is doing fine," Vance said at a news conference today. "She has been treated and she'll be instrumental in this investigation."
In addition, sources told ABC News authorities are investigating reports that Lanza had an altercation at the school just days before the attack.
Vance said that Lanza forced his way into the school, but did not say how.
Evidence emerged today that Lanza's rampage began in the office of school principal Dawn Hochsprung while the school intercom was on. It's not clear whether it was turned on to alert the school or whether it was on for morning announcements, but the principal's screams and the cries of children heard throughout the school gave teachers time to take precautions to protect their children.
Hochsprung was among those killed in the Friday morning killing spree.
Authorities have fanned out to New Jersey, Connecticut and Massachusetts to interview Lanza's relatives, ABC News has learned.
According to sources, Lanza shot his mother in the face, then left his house armed with at least two semi-automatic handguns, a Glock and a Sig Sauer, and a semi-automatic rifle. He was also wearing a bulletproof vest.
Newtown Police Hunt for Motive in School Massacre
Lanza then drove to the elementary school and continued his rampage, authorities said.
It appeared that Lanza died from what was believed to be a self-inflicted gunshot wound.
"Evil visited this community today," Gov. Dan Malloy said at a news conference Friday evening.
This is the second worst mass shooting in U.S. history, exceeded only by the Virginia Tech shooting in 2007 when 32 were killed before the shooter turned the gun on himself. The carnage in Connecticut exceeded the 1999 Columbine High School shooting in which 13 died and 24 were injured.
Friday's shooting came three days after masked gunman Jacob Roberts opened fire in a busy Oregon mall, killing two before turning the gun on himself.
The Connecticut shooting occurred at the Sandy Hook Elementary School, which includes 450 students in grades K-4. The town is located about 12 miles east of Danbury, Conn.
ABC News' Michael S. James contributed to this report.