The father contends his son was set up by two other security agents who have been jailed on charges they helped carry out the assassination.
In 1991, he said, the Kurds jailed him on charges of working with Turkish intelligence.
He has acknowledged jailing people on such charges as sabotage and trying to overthrow his Government.
Ms. Stewart was convicted and jailed last year on charges of lying to investigators about a stock trade.
He had recently been jailed on charges of being an accomplice in another man's murder.
The two others had already been jailed, on unrelated charges.
Four other men were jailed on related charges.
Many of its politicians, jailed on criminal charges, are campaigning from behind bars.
The number of suspects jailed on charges connected to the killings has slipped to just under 125,000, from 130,000 several years ago.
Most have relatives jailed on charges of collaborating with rebel groups.
