A downtown Toronto school spent the afternoon in lockdown after a gunshot was fired inside the building, leading to a massive police response.

By 7:30 p.m. Thursday, police had a third suspect in custody, CTV News reported. That person did not surrender. He has been taken to 14 Division for processing and questioning.

The incident began at 1 p.m. Police received a call from Central Tech high school, near Harbord and Bathurst Streets. "Police came in and said, 'We're going to lockdown, Sheryl,'" principal Sheryl Freeman told reporters after the lockdown ended.

When officers arrived, they found a bullet casing near a stairwell.

Emergency Task Force officers were dispatched, as well as a canine unit, but police say this is part of standard procedure for such calls.

Toronto Police Const. Tony Vella told reporters that the area around the school was blocked off as a safety measure.

Two other nearby schools, Harbord Collegiate and King Edward Public School, were put into "hold and secure" mode as a precaution. The hold-and-secure order for those two schools was lifted shortly before 4 p.m.

Police initially had two people in custody, and are searched for a third.

The third individual should be considered as armed and dangerous, Vella said.

He described the youth as being a 17-year-old non-white male who is 5'7" and 125 lbs. The person of interest was last seen wearing a black shirt with red trim.

Officers have not yet recovered a firearm.

Freeman said they have handed over video from the school's security cameras.

Night courses for the school will be taking place as scheduled.