A man holds a candle during a prayer vigil at Calvary Lutheran Church in Murrysville, Pa., for victims of the school stabbing. (Shannon Stapleton/Reuters)

A day after a student wielding two knives injured 21 students and a security guard at a high school near Pittsburgh, police said they didn’t know yet what caused the violent spree.

“Our investigation will be lengthy,” Thomas Seefeld, the Murrysville police chief, said in a news conference Thursday morning. “It will take time until we can get everything put together to solidify the case.”

The violent attacks occurred Wednesday inside Franklin Regional Senior High School in Murrysville, Pa., about half an hour east of Pittsburgh. At least five students were critically injured in the chaotic, nightmarish scene unfolding right as the school day was getting underway.

A student holding a knife in each hand “just went down the hall, swinging the two knives he had,” Seefeld said. It appears that many of the injuries were from slashes rather than direct stabbings, Seefeld said.

The suspect, identified by officials as 16-year-old Alex Hribal, a sophomore at the school, is being held at a juvenile detention center. He is being charged as an adult.

Seefeld said that the security protocols surrounding the school would be reviewed, but he reiterated something he had said in the hours after the attack Wednesday: The outcome could have been even more grave.

“As tragic as this was, it could have been worse,” he said Thursday.

He has pointed to the actions of Sam King, an assistant principal at the school who tackled the attacker, and also praised the actions of a person who pulled the fire alarm during the attack. When police arrived, the suspect had already been handcuffed by the school resource officer, Seefeld said.

The high school will remain closed for a few days, according to the Franklin Regional School District, while elementary and middle schools reopened Thursday.

But the school has been cleared of all evidence and turned back over to the school district, said Dan Stevens, a spokesman for Westmoreland County’s Department of Public Safety.