The rifle carried by the man who killed four RCMP officers on an Alberta farm last month couldn't be switched to fully automatic mode, police say.

James Roszko could not have sprayed the men with automatic gunfire but would have had to squeeze the trigger at least four times, the RCMP said Thursday.

Last week, the force confirmed that the 42-year-old had been carrying at least three guns, including a Heckler & Koch, Model 91 rifle that fired the fatal bullets.

James Roszko fired the fatal bullets from a Heckler & Koch, Model 91 rifle. (CP file photo / CTV)

At that time, officers didn't know whether it was capable of firing in a semi-automatic or fully automatic mode.

On Thursday, the RCMP said firearms specialists had tested the rifle and determined that it hadn't been modified to be fully automatic.

"With a fully automatic weapon, it takes one squeeze of the trigger and as long as that trigger is held, the weapon will continue to fire so long as there are rounds in the magazine," said RCMP Cpl. Wayne Oakes.

"With a semi-automatic, for each bullet that is fired, it requires a squeeze of the trigger."

Unlike a regular rifle, he said, "you don't have to cock, you don't have to action a bolt, you don't have to pump a lever, you just keep squeezing the trigger."

The 20-round magazine found with the rifle is illegal in Canada, but the RCMP could not say whether the weapon itself was prohibited.

The RCMP had earlier revealed that Roszko had been carrying at least two other guns, including a semi-automatic pistol in his waistband and another long gun over his shoulder.

Const. Peter Schiemann, 25, Const. Brock Myrol, 29, Const. Anthony Gordon, 28, and Const. Leo Johnston, 32, were gunned down on March 3 while guarding a 350-plant marijuana grow operation and stolen auto parts found on Roszko's farm.

Roszko, known to hate the police, killed the officers and then himself.