Nineteen students and an adult were injured, at least seven of them seriously, when a teenage boy dressed in black and armed with knives attacked students and staff at a high school in Pennsylvania on Wednesday, officials said.

"A critical incident has occurred at the high school. All elementary schools are canceled, the middle school and high school students are secure," said the Westmoreland County school district in a statement posted on its Web site.

A 16-year-old boy was arrested after the incident at Franklin Regional High School in Murrysville, 40 kilometers (25 miles) east of Pittsburgh, and police said he is the attacker and is a student at the school, the spokesman for the county's Emergency Department, Dan Stevens, told reporters.

He added that the attack began shortly before the first class of the day and the attacker wielded two knives.

The suspect was later questioned by police and taken to a hospital for minor head injuries.

Initial reports broadcast by local television say that one person went through the high school's halls stabbing students he encountered.

Several of the injured were taken to medical centers by helicopter with eight of them being transported to the Forbes Regional Hospital in Monroeville with stab wounds, Pittsburg's Channel 4 TV reported.

Forbes Hospital physician Mark Rubino said that three of the students required surgery and two are in critical condition. Other medical authorities said that at least seven people were seriously injured with cuts and penetrating wounds in the chest and abdomen.

Murrysville police chief Thomas Seefeld said that the school's assistant principal, Sam King, was the person who grabbed and subdued the knife-wielding teen but not before a security guard who had confronted the attacker was stabbed in the abdomen.

Franklin High School has about 1,200 students. EFE