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No evidence has yet surfaced to indicate that a 16-year-old boy charged in a stabbing rampage at a Pennsylvania high school was bullied or targeted any particular student, police said Friday.

Murrysville Police Chief Thomas Seefeld says a motive for the attack Wednesday on other students at Franklin Regional High School near Pittsburgh remains unknown. Reconstructing exactly what happened remains difficult because suspect Alex Hribal isn’t cooperating with authorities.

"At this point I don't have anybody that, you know, was targeted," Seefeld said. "I know the issue of bullying has been brought up but his attorney has even said ... that bullying is not part of this and we have no evidence or reason to believe that it is."

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Hribal allegedly stabbed or slashing 21 students and a security guard. Eight students remained hospitalized on Friday, including four in critical condition after one was downgraded.

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Hribal suggested he wanted to die when he was apprehended, according to a prosecutor. Seefeld said Hribal indicated "he wanted someone to kill him."

A witness said Thursday that Hribal had a “blank look” on his face during the five-minute rampage.

"He was just kind of looking like he always does, not smiling, not scowling or frowning," 16-year-old Mia Meixner said.

Hribal, a sophomore at the school, was charged with four counts of attempted homicide, 21 counts of aggravated assault and one count of possessing a prohibited weapon on school property, according to a criminal complaint released by Westmoreland County District Attorney John Peck.

Seefeld has said investigators were checking reports of a threatening phone call between Hribal and another student the night before. He didn't say whether the suspect received or made the call.

"I don't know of anything right now, you know, that would reveal any motive," he said. "That's the big question out there."

The stabbing spree occurred just before class Wednesday. One student said he thought the melee was a fistfight, but said it turned out that Hribal was not punching, but stabbing his victim in the belly.

Meixner said the boy who was tackled by Hribal tried to fight back, then, when his assailant got off him, stood up and lifted his shirt to reveal a midsection covered in blood.

"He had his shirt pulled up and he was screaming, `Help! Help!"' said another witness, Michael Float, 18. "He had a stab wound right at the top right of his stomach, blood pouring down."

As students rushed to the boy's aid, the attacker slashed another student, Nate Moore, before taking off around a bend.

"It was really fast. It felt like he hit me with a wet rag because I felt the blood splash on my face. It spurted up on my forehead," said Moore, whose gashed right cheek required 11 stitches.

The suspect then ran down about 200 feet of hallway, slashing and stabbing other students with kitchen knives about 8 to 10 inches long, police said. The assault touched off a "stampede of kids" yelling, "Run! Get out of here! Someone has a knife!" according to Meixner.

Assistant Principal Sam King heard the commotion and found a chaotic scene in the blood-soaked hall. King then saw Hribal stab a security guard, who leaned against the wall, bleeding from his stomach, according to a police affidavit. King tackled Hribal and kept him on the floor until a school police officer handcuffed him.

"There are a number of heroes in this day. Many of them are students," Gov. Tom Corbett said in a visit to the stricken town.

He also commended cafeteria workers, teachers and teacher's aides, who put themselves at risk to help others.

Meixner and Moore called the attacker a shy and quiet boy who largely kept to himself, but they said he was not an outcast and they saw no indication before the attack that he might be violent.

Student Alicia Graham said she was in the school's library when the initial fire alarm went off.

"Everyone was screaming and people were being trampled over," she told FoxNews.com on Wednesday. "It was a very bad scene. I was very scared. I didn’t know what was happening. I thought it was a drill."

FoxNews.com's Joshua Rhett Miller and The Associated Press contributed to this report.