Three men who appear to have been making a citizens’ arrest might now be facing second-degree murder charges.

A 34-year-old man was killed on Tuesday night after he allegedly committed a robbery in an East End depanneur.

Police say the alleged thief fled the store, running down an alleyway.

That's when the three key witnesses caught up with him. A call was made at 8:45 p.m. regarding a fight on Nicolet St. near Ontario St.

“When police officers attended the scene they found three men who were controlling or in a fight with another person,” explained Montreal police spokesperson Const. Raphael Bergeron.

The man was on the ground, unconscious. He was taken to hospital, where he succumbed to his injuries overnight.

The investigation has been taken over by SPVM’s Major Crime Unit. Three men who officers said were grappling with the victim are being treated as important witnesses and met with investigators on Wednesday morning. Bergeron said police are still trying to determine precisely how the three men were involved in the incident.

Bergeron said an autopsy would be conducted to determine the exact cause of death, but said it appears no weapon was involved. They are treating the case as a homicide.

The three men, aged 37, 48 and 78 could now face charges of second-degree murder.

“Obviously if (a citizen’s arrest) goes bad or someone gets injured or anything like that, if you didn't use force properly you can also… be charged,” said Bergeron.

Defence attorney Eric Sutton said citizen's arrests, while rare, are not illegal.

"They have to have witnessed a crime or have reasonable or probable grounds to believe a crime has been committed and that the victim is seeking to effect an arrest," he said. "If they have reasons the person is honest and is a victim, they could effect an arrest."

However, he warned that those wishing to make a citizen's arrest must be careful with the use of force.

"The power to arrest doesn't include the power to use force to detain the person unless you've witnessed the crime yourself," he said. "In this case, from what we understand, they may not have witnessed the crime. We don't know that at this point. If they did witness the crime, they can use reasonable force to detain the suspect. Where you use deadly force, it will be seen as unreasonable. They are not police officers, they are simply citizens, and they have to be reasonable."

It's Montreal's 29th homicide of 2018.