NEWTOWN, Conn. — The gunman who coldly and methodically shot and killed 20 children and six adults at Sandy Hook Elementary School forced his way onto school grounds, authorities said Saturday morning.

Lt. Paul Vance of the Connecticut State Police said the details of the Friday morning mass shooting — the second-deadliest in U.S. history — remain hazy, and he would not comment on what prompted 20-year-old Adam Lanza to stage the attack on the school.