By M. Alex Johnson, NBC News

NEWTOWN, CT. (NBC) -- The first that some pupils heard of the mass shootings Friday at their Connecticut elementary school was when screams started coming over the intercom system, according to accounts from the scene.

Eighteen children, all ages 5 to 10, and eight other people were killed when a masked gunman opened fire inside Sandy Hook elementary School in Newtown, Conn., authorities said. The gunman was later found dead, a federal law enforcement official told NBC News.

The brother of a little girl who made it out of the school safely told NBC 30 of Connecticut that his sister told him that her class knew something was wrong "when she heard screams — screams were coming over the intercom and the school went on lockdown."

As the children were being led out of the school, state troopers and FBI agents told them to "hold hands and close their eyes until they outside," said the unidentified young man, "so obviously what was in there must have been very gruesome."

"It truly is disgusting. It's sickening," he said. "Unfortunately, it's a growing trend in the United States."