Five inmates were sent to hospital after a melee erupted in the dining room of Edmonton's maximum-security prison during breakfast on Wednesday.
At least one of the inmates has been critically wounded but no prison staff were injured.
Edmonton Institution spokesman Rick Dyhm told The Canadian Press a homemade weapon was found at the scene.
The violence "caught everyone off guard'' and may have been gang-related, said Dyhm.
Dyhm also confirmed that the institution had recently been locked down for four days after the discovery of a homemade weapon. Several other weapons were found during that lockdown.
A STARS air ambulance had been dispatched at one point for the most injured patient, but officials there wanted three guards to accompany this particular prisoner, Bart Goemans, the service's manager of communications, told CTV.ca.
"Our craft doesn't have that capacity," he said, saying there would be one or two spare seats left once medical personnel are accommodated.
"We were unable to accept the mission. It was basically a security and a capacity issue."
All five inmates ended up being transported by ground ambulance as their injuries were severe enough that they could not be treated in the prison medical unit.
Jeff Campbell of the Correctional Service of Canada told CTV.ca that the incident unfolded during breakfast shortly after 8 a.m. local time.
"There was a major altercation involving several inmates," he said.
Prisoners have been locked in their cells, Campbell said, adding that is standard procedure in these cases.
The prison is secure and there is no wider unrest within the institution, he said.
Those prisoners who had to forgo breakfast are being fed in their cells, Campbell said.
"We'll be fully investigating this incident," he said, adding there's no word yet on a motive.
Edmonton Institution opened in 1978. The facility's recommended capacity is 221 inmates. Campbell put the current inmate count at 230.
"We have maximum-security inmates, so as a thumbnail sketch they would be young, violent, aggressive, impulsive, many involved in gang activity, many of them involved in the drug trade,'' Dyhm said.
"I can't remember the last time that we had an assault of this magnitude. We have assaults on a fairly regular basis.''
Edmonton police spokesman Jeff Wuite told Â鶹´«Ã½net said that during his tenure with the Edmonton police, which began in 2002, the "Max" has been relatively quiet.
With files from The Canadian Press
