NEW YORK/MURRYSVILLE, Pa: An American student stabbed 19 teenagers and a security guard in a bloody 30-minute rampage in the classrooms and hallways of a Pennsylvania high school on Wednesday, officials said.

The 16-year-old student holding two knives went on a stabbing rampage in the hallways of a Pittsburgh-area high school.

At least four students were critically injured in the shocking assault at Franklin Regional High School, said Thomas Seefeld, chief of police in Murrysville, an outer suburb of Pittsburgh.

The suspected attacker, who was not identified, was in police custody, Dan Stevens, deputy emergency management coordinator for the Westmoreland County, told reporters.

The attacks began around 7:13 a.m. local time in several classrooms and hallways as the classes were starting at Franklin Regional High School in Murrysville, 32 km east of Pittsburgh, officials said.

The school was evacuated quickly after a student or staff member pulled the fire alarm, said Murrysville Police Chief Tom Seefeld.

"What we teach is, the first thing you want the students to do is run," Seefeld said. "The fire alarm being pulled probably assisted with evacuating the school."

Seven students between the ages of 15 and 17 and one 60-year-old adult were admitted to Forbes Regional Hospital in Monroeville, Pennsylvania, with stab wounds.

Three of the patients were in surgery and all eight were listed in critical condition, hospital spokesman Jesse Miller said in a telephone interview.