NEWTOWN, CONN. - As many as a half-dozen first-graders may have survived Adam Lanza's shooting spree at Sandy Hook Elementary School because he stopped firing briefly, perhaps either to reload his rifle or because it jammed, according to law enforcement officials familiar with the events.

A source said the Bushmaster rifle that Lanza used in the shooting is at the State Police forensic laboratory undergoing several tests, including tests to determine whether it was jammed.

The children escaped from the first-grade classroom of teacher Victoria Soto, one of the six adults Lanza killed in the school after shooting his way through a glass door with the .223-caliber semiautomatic rifle the morning of Dec. 14.

Detectives obtained and began examining records Friday related to psychiatric care Lanza had received in an attempt to determine a motive.

Lanza killed 27 people -- 20 children and seven adults -- before shooting himself in the head as police began arriving at the school.

The officers arriving to Soto's classroom found that Lanza had shot her as well as special education teacher Anne Marie Murphy and six of Soto's students. Seven other children were found unharmed in a closet, apparently hidden by Soto when she heard shooting. The other students fled the room.

Based on initial statements from surviving children and the fact that unfired bullets from Lanza's rifle were found on the ground, detectives suspect that some students were able to run to safety when Lanza stopped firing, probably for a short time, the officials said.

It is possible that Lanza, who reloaded the rifle frequently, mishandled or dropped a magazine and unfired bullets fell to the floor, they said.

But it also is possible, they said, that the mechanism that fed bullets into the rifle jammed, causing Lanza to remove the magazine and clear the weapon. Unfired bullets could have fallen to the floor during that process as well, law enforcement officials said.

The six children who escaped Lanza's rampage ran to a private home a short distance from the school.

The substance of the children's statements, which are not entirely consistent, is that a piece of the weapon, probably a magazine holding live bullets, was dropped or fell to the floor. It is also possible that some children escaped while Lanza was shooting others.

State police are expected to wrap up work at the school and release it as a crime scene in the next few days. They still are trying to determine how many shots Lanza fired.

Lanza killed himself in Soto's classroom with one of the two pistols he carried into the building. Police found a loaded 20-round shotgun in the trunk of the car similar to what is known as a "street sweeper." Police believe that Lanza didn't bring it into the school because he couldn't carry all of the weapons and ammunition. Lanza, who was about 6 feet tall, weighed barely 110 pounds, law enforcement sources said.