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.

Vance, however, did say that detectives have found substantial evidence at Lanza's home that he hopes will "paint a complete picture" of the gunman and his motive.

Authorities are also sure that Lanza was not admitted into the school, but forced his way into the building. Because the school had recently installed a high-tech security system, many wondered if the shooter was granted access to the building, but Vance emphatically denied those rumors.

Lt. George Sinko with the Newtown Police Department said it was possible Lanza shattered a glass panel next to the front door to gain entrance, but police tactical units also broke windows when they swarmed onto the scene.

Police believe Lanza entered the school around 9:30 a.m. Friday and opened fire on a classroom of young students. Mary Ann Jacob, a library clerk at the school, said she heard a weird noise over the intercom and called the front desk, at which time she was informed there was a shooting.

Jacob said she called out "lockdown," and students and teachers in the library took cover, as they were trained to during routine drills. Jacob eventually moved the students into a storage room and locked the door, putting file cabinets in front to protect the students from possible gun fire.

The students, a fourth-grade class, were handed crayons and paper to pass the time, and were scared but calm, Jacob said.

Lanza killed 20 students described as between the ages of 5 and 10 and six adult staff including Principal Dawn Hochsprung. The gunman also took his own life.

After the shooting, police learned Lanza also killed his mother, Nancy, at their Newtown residence. In all 28 were killed, including the shooter. Only the massacre at Virginia Tech in 2007 that took the lives of 32 people ended in more deaths.

There are conflicting reports of whether Nancy Lanza worked at the school. Jacob said she had never heard of the Adam or Nancy Lanza in her five years on staff.

One casualty was taken to a local hospital where she is "doing fine," Vance said. Police are optimistic the victim will be "instrumental" in helping them piece together the days' events, he added.

Police hope to release the names of the victims later Saturday. Families of the victims have each been assigned a state trooper and are asking for complete privacy.