Detectives in St. Petersburg, Fla., are trying to determine why a man from Davis, Calif., who was visiting relatives for the holidays, allegedly suffocated his 83-year-old father with a pillow and claimed it was self-defense.

"We're still working on a motive. We have no idea why he did what he did," said Jennifer Dawkins, a spokeswoman with the St. Petersburg Police Department.

The suspect, Stephen Coffeen, 41, is a senior analyst with a prominent water engineering firm in Davis.

He is a married homeowner in the upscale college town, with no criminal background or outward signs of domestic troubles or mental instability, according to investigators, neighbors and co-workers.

His father, Robert Coffeen, was in good physical health and sharp of mind, undermining the idea that it may have been a mercy killing, police said.

With a motive still a mystery, Coffeen is being held without bail in the Pinellas County jail on a charge of second-degree murder.

He was arrested Friday at his father's home in a middle-class area of north St. Petersburg after his brother called police and Coffeen confessed, police said.

His father's body was still on the living room floor, a pillow covering his face, when emergency crews arrived on Friday afternoon, Dawkins said.

Stephen Coffeen made a spontaneous statement, Dawkins said, admitting the killing but insisting, without explanation, that it was self-defense.

"He was cool, calm, cooperative," Dawkins said. "All he said was, 'I suffocated my dad in self-defense.' There was no elaboration whatsoever."

"Detectives don't really believe that," she said, adding they are working to find out what really happened.

Dawkins said investigators are proceeding on the assumption that the killing was not premeditated. That's why Coffeen was booked on a second-degree murder charge, she said.

A Florida judge on Tuesday ordered Coffeen to undergo a psychological evaluation, court records show.

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