MURRYSVILLE, PA (CNN) - At least 20 students were injured Wednesday morning in a stabbing incident at Franklin Regional Senior High School in the Pittsburgh area, said Dan Stevens, an emergency management agency spokesman.
The stabbings happened in classrooms and a hallway, he said, adding that a suspect -- a male student -- was in custody.
Four people were flown to hospitals by helicopter, Stevens said.
Dr. Chris Kaufmann at Forbes Regional Hospital in Monroeville, Pennsylvania, spoke to CNN on air. He said the facility is treating seven teens with stab wounds to their torso, abdomen, chest and back areas. He said that the injuries were "quite serious" and that "some are clearly life-threatening." An adult is also being treated there, he said.
He said two of the victims being treated at Forbes Regional were sent to surgery almost immediately on arrival. Those two patients had low blood pressure, he said. A third was being prepared for surgery Wednesday morning, and the rest were being evaluated to see whether surgery was necessary, he said.
A message on the Franklin Regional School District's website said all of its elementary schools were closed after the incident, and "the middle school and high school students are secure."
Information on what led to the stabbings and the conditions of the injured were not immediately released.
The students who were hurt range in age from 14 to 17, Stevens said.
The school is in Murrysville, about 15 miles east of downtown Pittsburgh.
Later Wednesday morning, students were being released to their parents, Stevens said. Shortly before 10 a.m. ET, CNN affiliate KDKA reported that some parents were beginning to be reunited with their children.
Bill Rehkopf, a KDKA radio host and Franklin Regional High School graduate, reported on air that he was shocked by the stabbings.
He kept thinking, "It doesn't happen here, it can't happen here," he said.
He said he was seeing parents showing up at the school and an increasing media presence. Parents appear to be calm, he said. Another KDKA reporter said she spoke with a parent who said she received a cell phone call from her daughter, who told her mother that "something bad" happened and that she needed to be picked up.
A media briefing is scheduled for 10:30 a.m. ET at Forbes Regional Hospital.
CNN first learned of the stabbings on Twitter.