An American student stabbed 19 teenagers and a security guardin a bloody 30-minute rampage in the classrooms and hallways of a Pennsylvaniahigh school on Wednesday, officials said.At least four students were critically injured in the shocking assault at FranklinRegional High School, said Thomas Seefeld, chief of police in Murrysville, an outersuburb of Pittsburgh.The 16-year-old boy walked down a hallway flashing two knives, stabbing fellowpupils and the guard as they began arriving for what should have been a routineschool day, Seefeld said.Nineteen students and one male security guard were injured, mostly in the chest andstomach, officials confirmed.The school principal and another member of staff tried to overpower the suspectwho was then arrested, Seefeld said.Officials said the assault began at 7:13 am (1213 GMT) was over by 7:45 am.It was not immediately clear what prompted the assailant, who is himself beingtreated for a hand injury, to attack.Doctor Mark Rubino at Forbes Regional Hospital, which is treating eight of thevictims, said he expected everyone to survive despite what he called "deeppenetrating" stab wounds.The knife wounds, most of them to the lower abdomen, caused significant injuriesto internal organs, Rubino said.Three of the victims were still in surgery and the other five were still beingevaluated, he told reporters."Three of the patients had severe injuries and are still in the operating room rightnow. And two of them are in critical condition, but they have stabilized," Rubinosaid.The other five that we had are still being evaluated, of whom one or two mayrequire further surgery, he added.Dan Stevens, a spokesman for Westmoreland County emergency management, toldAFP that the teenage victims are aged 14 to 17.They were attacked in "numerous classrooms and hallways" of the school, Stevensadded.Police said the victims were found in multiple locations because they were followingthe correct drill to run as far away as possible in the event of an emergency.Wednesday's attack comes after a long and frequent line of US school shootings thathave inflamed a nationwide debate over gun control in the United States.But even the December 2012 massacre at Sandy Hook Elementary in Newtown,Connecticut, where 20 young children and six adults were shot dead, ultimatelyfailed to tighten gun ownership rules.Seefeld praised school staff, who worked in close coordination with police and who were well versed in emergency procedure."It is my opinion that today as unfortunate as it is... I think that it could have been alot worse if there was not immediate interaction that occurred," he told reporters.The school has since been evacuated.A statement on the website of the Franklin Regional School District said a "criticalincident" had occurred."All elementary schools are cancelled, the middle school and high school studentsare secure," the statement said.