A man armed with two semiautomatic pistols killed 20 children at an elementary school Friday in a rampage that left 27 people dead and one injured in this suburban town before he took his own life, authorities said.

Police said the suspect, 20-year-old Adam Lanza, entered Sandy Hook Elementary School and began firing shortly after 9:30 a.m., concentrating on two classrooms. It was the second-worst campus shooting in U.S. history.

No motive was cited by authorities. State police said they found the body of someone who lived with the suspect at a house nearby. Mr. Lanza’s mother was among the victims.

Diane Day, a therapist at the school, was sitting with the principal, a parent and other staff members for a meeting when she heard gunshots.

“At first we heard a bunch of kids scream, and then it was just quiet and all you could hear was the shooting,” Ms. Day said.

Principal Dawn Hochsprung and the school psychologist leapt from their seats and ran out to help, Ms. Day said. “They didn’t think twice about confronting or seeing what was going on,” she said.