Before events at the school

At some point before he went to the school, investigators believe Adam Lanza, 20, killed his mother, Nancy Lanza.

He grabbed three guns from the house -- a semi-automatic AR-15 assault rifle made by Bushmaster and pistols made by Glock and Sig Sauer -- and went to the elementary school wearing black fatigues and a military vest, according to a law enforcement official.

Classes were under way at the school. Approximately 700 students were present.

Earlier this year, the school principal, Dawn Lafferty Hochsprung, ordered a new security system installed that required visitors to be visibly identified and buzzed in. As part of the security system, the school locked its doors each day at 9:30 a.m.

The door was locked when the gunman arrived.

Authorities now know the gunman used "an assault weapon" to "literally (shoot) an entrance into the building," Connecticut Gov. Dannel Malloy said.