MURRYSVILLE, Pa. -- Detectives say 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.

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. Murrsyville 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 Monday that police have not definitively linked the calls to the 16-year-old suspect but were working to determine if they came from a phone he could have used.

Defense attorney Patrick Thomassey did not return calls Monday from The Associated Press. But he told the Pittsburgh Post-Gazette: "I don't know of those phone calls."

He previously told the AP that Hribal's parents weren't aware of any threatening calls.

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 Monday, 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 Monday, part of a three-day effort by school officials and a hired crisis management team to fully reopen the school for classes Wednesday.

On Tuesday, students and parents will get a chance to see that hallways have been cleaned up and other steps taken to return the school to normal.