An off-duty police detective providing security for a private party was shot to death, and a second detective was wounded.

A manhunt was under way Monday and a $50,000 reward was being offered in the search for information into the shooting of Donald R. Young (search), 43.

Young, a married father of two and a decorated detective, was shot three times from behind around 1 a.m. Sunday, authorities said. Detective John H. Bishop (search) was treated for a minor gunshot wound in his back and released.

The motive was not known.

Police Chief Gerry Whitman called it an ambush and said the officers never had a chance to defend themselves. Young and Bishop were off-duty but in uniform while providing security outside a rental hall often used for birthday and baptism parties.

Two other officers who were nearby rushed to the aid of the fallen detectives and saw a man fleeing with what appeared to be a weapon, authorities said.

"This was cold-blooded murder," Mayor John Hickenlooper (search) said in a statement. "And we will not rest until we bring those responsible to justice."

Young, a 12-year veteran, had worked with the vice-narcotics and domestic violence units. He was assigned to the fugitive-bomb squad when he was killed.

The last fatal shooting of a Denver officer was in 1997, when Bruce VanderJagt was gunned down by a robbery suspect who later killed himself.