Police are hunting for a suspect after three men were hit on the back of their heads in downtown Calgary, at least one of them with a piece of concrete.

The first of the apparently unprovoked attacks happened last Saturday at about 11:30 p.m. in the 700 block of Eighth Street S.W., police said in a news release.

The victim was taken to hospital in life-threatening condition, but has since been upgraded to stable condition.

In the second attack, a man was using a payphone at the Safeway on 11th Avenue S.W. just after midnight on Monday when he was struck in the back of the head with a piece of concrete, police say.

He was transported to hospital with life-threatening injuries, but has also since been upgraded to stable condition.

The third attack happened early Tuesday morning near the intersection of Eighth Avenue and Ninth Street S.W..

Two police officers on patrol discovered the victim unconscious and in serious medical distress.

Investigators believe he was also hit over the back of the head with a hard object. He was taken to hospital in stable condition.

No known connection

Investigators do not think the three men had any connection to one another, but they don't know if any of them had some kind of prior involvement with the suspect.

"All of the victims are vulnerable people who lead a bit of a higher-risk lifestyle that may be more susceptible to this type of attack," said Insp. Ryan Ayliffe.

He said two of the victims have so far been reluctant to cooperate with the investigation.

The suspect was caught on a surveillance camera at one of the attack sites.

He is a black male and was wearing a white T-shirt over a dark-coloured hoodie, tan pants, and dark-coloured shoes with white soles.

Beat patrols in the area, including the mountain bike units, will be beefed up in the overnight periods, Ayliffe said.

"We feel confident we will find this individual," he said.

Anyone with information about these incidents, or who can identify the suspect, is asked to contact Calgary police at 403-266-1234.