The sounds of gunshots and chilling screams echoed through the halls of a Connecticut elementary school Friday morning as a gunman opened fire on young children and adults in the building.

The 20-year-old shooter killed 26 people in the school, including 20 children, before committing suicide Friday afternoon at Sandy Hook Elementary school, police said. The gunman had earlier killed his mother at the home where they lived together, a police source said.

Survivors of the massacre described teachers locking doors and ordering the children to huddle in the corner or hide in closets after hearing shots.

A custodian ran through the halls, warning of a gunman on the loose, and someone switched on the intercom, alerting people in the building to the attack — and perhaps saving many lives — by letting them hear the hysteria going on in the school office, said teacher Theodore Varga.

"He said, 'Guys! Get Down! Hide,'" Varga said. "So he was actually a hero."

Varga was in a meeting with other fourth-grade teachers when he heard the gunfire and the intercom switch on.

"Everyone in the school was listening to the terror that was transpiring," he said.

It could not be confirmed whether the custodian survived the shooting.

'Why did those men want to kill us, mommy?'

Lisa Terifay's two children are in the first and fourth grades at Sandy Hook elementary.

"The told me all about it as soon as we were able to leave the school," Terifay told CBC's Ian Hanomansing by telephone. "Since we've been home they just really kind of don't want to think about it so we've really just been watching cartoons."

She said her youngest had turned to her while watching television and asked: "Why did those men want to kill us, mommy?"

Although she wasn't sure how much her two children had seen, she said they also heard the gunshots and screams that echoed over the school's intercom.

Terifay said she was thanking God they were still alive, but feared for the days and weeks ahead as her children come to grips with the traumatic incident.

Teacher shot in front of students

Robert Licata said his six-year-old son was in class when the gunman burst in and shot the teacher.     

"That's when my son grabbed a bunch of his friends and ran out the door," he said. "He was very brave. He waited for his friends."     

He said the shooter didn't utter a word.    

Stephen Delgiadice said his eight-year-old daughter was in the school and heard two big bangs. Teachers told her to get in a corner, he said.     

"It's alarming, especially in Newtown, Conn., which we always thought was the safest place in America," he said.

Ran to find his sister     

Mergim Bajraliu, 17, heard the gunshots echo from his home and ran to check on his nine-year-old sister at the school. He said his sister, who was fine, heard a scream come over the intercom at one point. He said teachers were shaking and crying as they came out of the building.     

"Everyone was just traumatized," he said.

Mary Pendergast, who lives close to the school, said her nine-year-old nephew was in the school at the time of the shooting but wasn't hurt after his music teacher helped him take cover in a closet.

'It's sheer terror,' says father

Richard Wilford's seven-year-old son, Richie, is in the second grade at the school. His son told him that he heard a noise that "sounded like what he described as cans falling."     

The boy told him a teacher went out to check on the noise, came back in, locked the door and had the kids huddle up in the corner until police arrived.     

"There's no words," Wilford said. "It's sheer terror, a sense of imminent danger, to get to your child and be there to protect him."

Tracy Hoekenga's second grader son Matthew said a teacher ordered students to their cubbies and a police officer came and told them to line up and close their eyes.

"They said there could be bad stuff. So we closed our eyes and we went out. When we opened our eyes, we saw a lot of broken glass and blood on the ground."

With files from CBC News