Montreal police say they have arrested four people following Thursday's multiple smoke bombing attacks that shut down the city's metro system and stranded hundreds of thousands of people.

The four people turned themselves in to police late Thursday afternoon, accompanied by their lawyers.

Police questioned the four – three women and a man – to determine charges.

"That could be mischief, but we'll see what else it could be," said police Sgt. Ian Lafrenière.

He wouldn't confirm that they are the same people as those captured in eyewitness images, but said the four were wanted in connection with the smoke bomb incident at the Lionel-Groulx station.

Police are also mum on which police precinct the suspects turned themselves in.

"Which location, we'll keep that for ourselves," Lafrenière said. "I don't want a protest in front of the building."

Police aren't sure how many people overall may have been involved in Thursday's attacks, but are certain it is more than four.

"The three events took place simulaneously, so it's hard to believe the four same individuals would have committed that," Lafrenière said.

Three smoke bombs were set off at three separate metro stations at the height of Montreal's rush hour, forcing a complete shutdown of the underground commuter system.

Several hours later, police released images of the suspects captured by witnesses. The three women and one man were captured on camera inside metro cars.

On Friday, a former classmate of three of the suspects who spoke with CBC identified three of the people that appear in the photos.

Friday morning, police searched a home on Avenue Letourneux in the city's east end. They would only say a "police operation" was underway at the three storey apartment building and would not confirm it was connected to the smoke bomb attack.

Metro attacks condemned

However, according to Radio-Canada, the location being searched is linked to two of the suspects in Thursday's events.

Police have also not confirmed if the metro attacks were linked to the anti-tuition hike movement. The events were immediately condemned by provincial politicians and local public security officials.

Montreal police said several witnesses came forward after the metro shut down and offered information to investigators.

"I guess people were shocked about what happened yesterday," Sgt. Ian Lafrenière said. "A lot of people were touched by that – either they were stuck in the metro or stuck in traffic because yesterday morning, that was a nightmare in terms of traffic."

He said investigators have been working to confirm all the information that came in. But, investigators are revealing little about what they've learned so far.

"In police we say, 'We don't want to know, we want to prove it,'" he said. "So that's why we need more time."