A student armed with a pair of knives wounded 20 people Wednesday morning in a rampage at a high school in Western Pennsylvania, officials said. Four students were reported in critical condition.

A 16-year-old was in custody following the attack at Franklin Regional High School in Murrysville, Westmoreland County, 15 miles east of Pittsburgh.

The victims included 19 students and a security guard, officials said.

Dan Stevens, Westmoreland County's deputy emergency management coordinator, said the student victims ranged in age from 14 to 17.

Four were airlifted to area hospitals and officials said they were in critical condition.

Officials said the suspect was armed with two knives and the victims were either slashed or stabbed, the Pittsburgh Post-Gazette (www.post-gazette.com) reported.

Murrysville Police Chief Thomas Seefield said the motive was unclear.

Stevens said a school security officer called 911 at 7:13 a.m. to call for medical help. The rampage unfolded before school started and as students were arriving for the day, he said.

A girl reportedly pulled a fire alarm to evacuate the school when she saw what was happening.

A witness said the suspect was tackled by Assistant Principal Sam King, according to the Post-Gazette.

Although parents were asked to stay away, they converged on the school seeking information.

The school district said all students were moved to a secure location at a nearby Middle School.