(Adds details on attacker, injuries, quotes)
By Elizabeth Daley
MURRYSVILLE, Pa., April 9 (Reuters) - A 16-year-old student holding two knives went on a stabbing rampage in the hallways of a Pittsburgh-area high school early on Wednesday, injuring 20 people, at least nine of them seriously, officials said.
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. EDT (1113 GMT)in several classrooms and hallways as the classes were starting at Franklin Regional High School in Murrysville, 20 miles (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.
All three undergoing surgery had been stabbed in the chest and abdomen, said Chris Kauffman, director of trauma at Forbes Regional Hospital.
"Patients who are stabbed in the abdomen and chest by definition have life-threatening injuries," Kauffman said.
A ninth patient who had arrived at the hospital was air-lifted to another facility, Miller said.
A 15-year-old girl who had been transported to another area hospital, Allegheny General, was discharged after being treated for "superficial" stab wounds, a hospital spokeswoman said.
Zak Amsler, a 17-year-old junior at the school, said the attack occurred just before his first class was scheduled to begin.
"I saw a girl with blood running out of her sleeve," Amsler said as he waited to pick up his younger sister, a student at the nearby middle school. "It was pretty mind-blowing."
Stevens, the county official, noted that the scene had been contained. "The students that are here are safe," he told reporters.
Pennsylvania Governor Tom Corbett said he had ordered state police to help local law enforcement respond to the incident.
"I was shocked and saddened upon learning of the events that occurred this morning as students arrived at Franklin Regional High School," Corbett said. "As a parent and grandparent, I can think of nothing more distressing than senseless violence against children."
The school district posted an emergency message on its website after the attack.
"A critical incident has occurred at the high school," the message said. "All elementary schools are canceled, the middle school and high school students are secure."
A long line of school buses was observed outside the school on Wednesday morning to take students to another elementary school nearby to be reunited with their families. (Additional reporting by Dave Warner in Philadelphia, Brendan O'Brien in Milwaukee and Jonathan Allen in New York; Writing by Scott Malone; editing by Gunna Dickson)