MONTREAL - For the second time since Monday, Montreal's metro system was the target of vandals on Wednesday morning as a smoke bomb was set off inside the Green line tunnel.

Montreal police received a call at 7:20 a.m. reporting that smoke was in the tunnel between the Beaudry and Papineau stations, and service on the Green line was interrupted as they searched for a smoke bomb.

The service interruption led to delays between Lionel-Groulx and Honore-Beaugrand stations, impacting the majority of the metro's second longest line.

According to Montreal police spokesman Daniel Fortier, the interruption was necessary to recycle the air between the two stations.

Shrek the dog from the police K9 unit helped officers comb the tunnel and find the device. Service was restored by 8:20 a.m. with police carrying the device—described as "homemade"—out of the metro in a brown paper bag.

Standing outside of Beaudry station, a man handing out newspapers told CTV News that he saw men wearing masks run out of the metro earlier in the morning.

On Monday, bags of bricks were thrown onto the tracks during the morning rush hour, causing long delays on most of the metro system. Emergency brakes were also triggered and smoke bombs were thrown later that day, causing further delays.