UPDATE, 12:40p ET: According to FoxNews.com, one female student described the suspect, who has not been publicly identified.

Alicia Graham characterized the 16-year-old suspect as a "small, skinny" classmate who didn't talk much. She didn't know him personally, but has acquaintances who do, she said.

The motive for the attack remains unclear. One witness told the Pittsburgh Post-Gazette that the suspect was tackled by assistant principal Sam King.

UPDATE, 12:00p ET: Jon Scott reported moments ago that one adult was among the 20 injuries. Two students remain in critical condition, but are expected to survive.

UPDATE, 11:15a ET: Officials say the suspect is a 16-year-old student. His name has not been released. He was taken away from the scene in handcuffs. He reportedly used two knives in the attack, according to reports.

UPDATE, 11:00a ET: Dr Mark Rubino of Forbes Regional Hospital briefed the media moments ago on the conditions of the critically injured students. He said three were still in surgery with severe injuries. Two of those patients are in critical condition but have stabilized.

Rubino said most wounds were to the right lower abdomen and right flank of the victims.

"That created some of the criticality of their wounds and the nature of their injuries," he said.

Watch his remarks above.

UPDATE, 10:45a ET: In the video below, you can hear the police audio from the moments after the attack. Also in the video, a student at the school was interviewed, saying a female friend was slashed in the face and a male friend was stabbed in the back. He said another friend's brother was tackled and stabbed several times.

UPDATE, 10:20a ET: Four students are in critical condition at a local hospital after a student went on a stabbing rampage at Franklin Regional High School in Murrysville, Pa. 

UPDATE, 10:10a ET: At least two students are in the operating room being treated for serious stab wounds to the torso, according to hospital officials. Another student is being wheeled in to the operating room. The additional students are receiving CT scans to see how serious their stab wounds are.

The suspect is reportedly a sophomore at Franklin Regional High School. He was taken into custody at the school and is being questioned by detectives. The FBI is on the scene assisting local authorities. 

UPDATE, 9:00a ET: Laura Ingle reported the latest details on America's Newsroom (video above.) Westmoreland County public safety spokesman Dan Stevens confirmed 20 students were injured, four seriously. The victims have been transported to several area hospitals. He said the stabbings happened in numerous classrooms and in hallways as students were arriving for the start of the school day.

Police and paramedics responded this morning to multiple stabbings inside a high school in Murrysville, Pennsylvania. According to a report from KDKA, the attack happened at a science wing of Franklin Regional High School, located about 17 miles east of Pittsburgh.

One male student is in custody.

Reports indicate there were as many as 20 victims, ranging in age from 14 to 60. None of the injuries are believed to be life-threatening at this point, according to authorities.

The school posted the following on its website:

"A critical incident has occurred at the high school. All elementary schools are cancelled, the middle school and high school students are secure. Additional information will be released as soon as possible. Please keep our campus clear of traffic."

Stay tuned to Fox News for more on this developing story.