EDMONTON -- A man who pleaded guilty to killing his own sons has taken the stand at a fatality inquiry into their deaths.

Jason Cardinal is serving a life sentence after pleading guilty to two counts of first-degree murder after the strangled bodies of six-year-old Caleb and three-year-old Gabriel were found in his Edmonton home just before Christmas five years ago.

Inquiry lawyer Peter Duckett told Cardinal would not be asking how they died, but why they died.

But Cardinal, who told court he suffers from mental illness, didn't answer most of the questions put to him, saying he couldn't remember.

He admitted that when child welfare workers apprehended the boys in 2010, he began only sporadically taking his medication.

By Monday afternoon, Cardinal refused to answer any questions at all, demanding the court treat him as a "hostile witness."

"I won't answer any more questions," he said. "This is about one person gaining from a tragedy."

The children's mother, Andrea Badger, has launched a lawsuit against the province and children's services, contending the deaths could have been prevented had the visits with the father been supervised.

Badger had primary custody and Cardinal was only allowed to see the boys on the weekends. But when she went to get them on the weekend they were found dead, no one answered the door.

Police later found the bodies of the boys and Cardinal, who was suffering from self-inflicted wounds.

Once Cardinal was removed from the court on Monday, his mother, Germaine Cardinal, took the stand.

She became emotional when asked about the last time she talked with her grandsons, saying they had talked about Santa Claus coming soon.

"That time for me as a grandmother is hard to remember and it is painful," she said. "No matter what is decided, it's not going to change the fact they're gone.

"No amount of money is going to make up for their loss."