DETROIT — A man who authorities say claims to have killed seven people for hire was ordered Monday to stand trial on a first-degree murder charge involving a police officer's wife.

At a preliminary hearing, a judge ruled there was sufficient evidence for Vincent Smothers, 27, and 20-year-old Marzell Black to stand trial in the killing of Rose Cobb. They pleaded not guilty after their arrests last month.

Smothers also was ordered to stand trial on first-degree murder charges involving the unrelated shooting deaths of two Detroit residents. Police say Smothers also confessed to four other homicides under investigation.

According to a statement read in court, Smother told police he swore off life as a hit man after ambushing Rose Cobb, who was killed the day after Christmas.

"My stomach was in knots" afterward, Smothers' statement said. "I felt like she was innocent."

Smothers's attorney, Gabi Deborah Silver, said after the hearing that she would continue to challenge the circumstances under which police obtained statements from Smothers and Black.

The statements said Detroit police Sgt. David Cobb offered the men $5,000 each to kill his 47-year-old wife. She was shot in the head four times while waiting in a minivan outside a drugstore while her husband shopped inside.

David Cobb was arrested April 20 but was released two days later when prosecutors said they didn't have enough information to authorize a murder warrant against him. Wayne County Prosecutor Kym Worthy said her office and police would continue to investigate.

Smothers' statement said he received $60,000 for killing seven people between August 2006 and the end of 2007.

"Cobb's wife was the last one I committed," the statement said. "Killing a cop's wife was tough. All the other ones were dope killings."

Smothers and Black remain held without bond in the Wayne County Jail. First-degree murder is punishable by life in prison with no possibility of parole.