Emergency personnel gather outside Franklin Regional High School in Murrysville, Pennsylvania, April 9, 2014. [Photo/Agencies]

'A shadow in the hallways'

Pennsylvania Governor Tom Corbett said he had ordered state police to help local law enforcement respond to the incident. The FBI also said it had deployed agents to work with local officials.

The high school will be closed for the next two to three days while police conduct an investigation, officials said.

"I don't know him really well, but he's always said 'hi'," said neighbor Lori Renda, 47, who said he played with her own children. "The family is so nice. Very, very nice."

As they were reunited with parents near the hilltop high school in the relatively affluent Pittsburgh suburb with a population of about 20,000, teens spoke about the incident.

Michael Float, an 18-year-old senior, described running down a staircase and finding a friend badly wounded.

"There was a pool of blood," Float said. "He had blood pouring down the right side of his stomach," and a teacher was applying pressure on the wound.

Zak Amsler, a 17-year-old junior, said the attack occurred just before his first class was about 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."

On Wednesday evening members of the community held candle-light vigils for the wounded.

Kaitlyn Pepper, holding a candle in front of Calvary Lutheran Church, said that she now attends another high school but knows the suspect from her time at Franklin.

"He was literally a shadow in the hallways. They said he had a girlfriend who goes to another school, but I don't know. No one really knew of him. But they know him today." Pepper said.