MURRYSVILLE, Pa. - They prepared for guns but faced knives instead.

In the era of Newtown and Virginia Tech and Columbine, danger and bloodshed came Wednesday to a Pittsburgh-area high school not at the end of a barrel but rather at the points of two flashing blades.

Just after dawn, police said, sophomore Alex Hribal rampaged through a wing of Murrysville High School in a scene straight from a horror movie, slashing and stabbing 21 students and a security guard with two 8-inch steel knives in a swift and apparently random attack that ended only when an administrator tackled him. Two students were treated for other injuries.

Within five minutes, a high school hallway was transformed into a bloody crime scene; sleepy students waiting for first period suddenly became victims of violence; and a slender, dark-haired 16-year-old described as quiet and studious, and looking younger than his years, emerged as the latest face of the national epidemic of school violence.

"When I saw a kid bleeding on the ground is when I realized this was really serious," student Hope Demont said. "It was absolutely mind-blowing."

Four of her peers remained in critical condition Wednesday evening, and Westmoreland County District Attorney John Peck said the incident could yet turn fatal.

"There is a question about whether that person will survive," Peck said during the assailant's evening arraignment before District Judge Charles Conway.

The teen, clad in a hospital gown after being treated for minor hand wounds, was charged as an adult with four counts of attempted homicide, 21 counts of aggravated assault, and one count of possessing a prohibited weapon on school property. He was denied bail.

Peck told the court that Hribal made some statements after school officials tackled him that indicated he wanted to die.

Defense attorney Patrick Thomassey described his client as a good student with no prior criminal record and no history of addiction to drugs or alcohol. He said his client was not a loner and interacted well with other students.

Thomassey described the incident as "bizarre" and asked for a mental evaluation to determine whether his client would be competent for an April 30 preliminary hearing.

"My prayers go out to everyone who was injured today and I hope they recover as quickly as possible," the youth's father, Harold Hribal, told WTAE-TV outside the family's home.

Despite several hours of interviewing the assailant, Murrysville Police Chief Thomas Seefeld said investigators had not uncovered a motive.

"We don't know what led up to this," Seefeld said. "We're praying and hoping the best for all the victims."

Gov. Corbett, appearing at a late-afternoon news conference after clearing his schedule and driving to Murrysville from Harrisburg, asked the question on everyone's minds: "What made him decide to get up today and do this?"

News of the attack attracted national attention and lit up social media as condolences flooded through cyberspace and at least two stabbing victims posted on Twitter pictures of themselves at trauma centers sporting bandages and wearing hospital gowns.

The incident also drew in federal law enforcement. The FBI was at the assailant's two-story siding-and-brick house on a cul-de-sac, executing search warrants, seizing computers and interviewing witnesses. And U.S. Attorney David Hickton appeared at a news conference and pledged his help. He noted one thing that was not seized: Hribal's cellphone; he said the teen did not have one.

Wednesday began in typical fashion as the high school's 1,222 students entered the building just after sunrise. But by 7:13 a.m., when police were first alerted to the chaos churning inside the school, terrified students would be fleeing for their lives.

It was one of the most vulnerable possible times for chaos.

"Once the students are in the building and in classes we can go into lockdown," school director Roberta Cook said. "But before school starts it's hard to completely secure the building."

The incident began in a classroom in the school's science wing when the assailant pulled out two knives and started slashing and stabbing fellow students, said Mark Drear, vice president of Capital Asset Protection, which provides security guards for the school.

Many students were still at their lockers. The students, some of them wounded, ran from the room with the assailant chasing them a few hundred feet down a hallway. He attacked other students along the way, Drear said.

One student who realized what was happening pulled a fire alarm to try to evacuate the school. That caused students who were in other classrooms to crowd into the hallway - an action that some credited with preventing a worse toll.

The relatively safe high school has no metal detectors, which is not unusual for a suburban school, according to Linda Hippert, executive director of the Allegheny Intermediate Unit.