MURRYSVILLE, Pa.

- Detectives believe a boy charged with stabbing 21 other students and a security guard at his Pittsburgh-area high school threatened two students by phone before the attack, according to a search warrant. Neither was one of the victims.

The warrant, obtained for the home of Alex Hribal hours after last Wednesday's rampage, said two students received "threatening phone messages and completed calls" from someone believed to be Hribal.

It said the threat of violence contained an expletive.

District Attorney John Peck said the two male students who got the calls were not among those stabbed or slashed in the attack, which occurred minutes before the start of classes at Franklin Regional High School.

Neither Peck nor the warrant say when the calls were made. Murrysville Police Chief Thomas Seefeld previously said investigators were looking into a threatening phone call the night before the assaults.

"The caller is believed to be the actor because of the subsequent conduct of the actor coming to school and attacking numerous individuals," the warrant said.

Seefeld noted yesterday that police had not definitively linked the calls to the 16-year-old suspect but were seeking phone records to determine whether they came from a phone he could have used.

Defense attorney Patrick Thomassey did not return calls for comment. But he told the Pittsburgh Post-Gazette, which first reported the contents of the search warrant: "I don't know of those phone calls."

He had previously told the Associated Press that Hribal's parents weren't aware of any threatening calls the night before the attacks.

The warrant indicates detectives seized three video-game systems and games, a wooden kitchen knife-holder, some notebook paper believed to contain Hribal's handwriting and two computers.

Four students remained hospitalized yesterday, three in critical condition. The fourth was upgraded to fair condition.

The high school's teachers attended a counseling session at a nearby church before returning to the school for the first time yesterday, part of a three-day effort by school officials and a hired crisis-management team to fully reopen the school for classes tomorrow.