Police say Christian Costello, 26, stabbed his father repeatedly, including at least once in the head, in front of horrified witnesses, after meeting his parents at a Braum's grill and ice cream shop.
"We know they both agreed to meet, they were both there willingly, but as far as what set him off to attack the commissioner, we still don't know," Oklahoma City police Capt. Paco Balderrama told reporters Monday.
It could take weeks to sort the situation out, he said. Among other things, some 17 witnesses need to be interviewed, he said.
Christian Costello was jailed Monday pending the filing of formal charges by prosecutors, he said.
The attack Sunday night started inside the restaurant and then continued outside.
Witness Patrick Spencer was driving nearby when he saw a wounded man outside the restaurant. He thought the man had been shot.
"We got a closer look and there was a guy chasing him with a knife," Spencer told CNN affiliate KFOR
. "It was shocking and dramatic."
He told the affiliate that the two men circled the building a few times before the attacker stabbed Costello in the head.
"He was pretty bloody, and the wife was trying to get him in their vehicle to get him away from the suspect." Spencer told the affiliate.
He said the attack finally ended when a witness in the parking lot used his vehicle to ram the assailant to the ground.
Balderrama said witnesses apparently used belts to restrain Christian Costello until authorities arrived.
He said he didn't know what size or kind of knife was used in the killing. He said Christian Costello had brought it with him to the restaurant.
Mark Costello became Oklahoma's labor commissioner in 2010. Before that, he founded the telephone software company AMCAT, according to his official website
He sold AMCAT in 2007 and founded several other technology companies.
Sen. James Lankford, a fellow Republican from Oklahoma, mourned the loss of Costello on Twitter.
"My family is stunned & deeply grieved to hear of the tragic death of Labor Commissioner Mark Costello," he tweeted. "What a tragedy."
Costello was married and had five children.