LOS ANGELES (AP) _ Racing legend Mickey Thompson and his wife may have been murdered in retaliation for Thompson's testimony in a murder trial that led to the conviction of their slain nephew's killer, a prosecutor said.

Orange County Deputy District Attorney Tom Goethals called the coincidence between the Thompson murders Wednesday and that of Scott Campbell six years ago ''absolutely incredible.''

''First, his nephew is murdered execution-style, then Thompson and his wife are hit execution-style. The connection just knocks you right out of your chair,'' Goethals said.

Authorities called the killings of the millionaire couple an assassination and said they had no motive for the crime and no suspects in custody Thursday.

Investigators are seeking a cyclist who abandoned his bicycle and flagged down motorists for a ride shortly after Thompson, 59, and his wife, Trudy, 41, were fatally shot at dawn Wednesday outside their home in Bradbury, 20 miles northeast of Los Angeles.

Detectives recovered the 12-speed bike and are checking for fingerprints and other clues, sheriff's Deputy Van Mosely said.

Sheriff's officials refused to say whether the man being sought was one of the cyclists seen fleeing the Thompsons' house, a third participant in the slayings, or had some other connection to the case.

Thompson, the first person to drive a car faster than 400 mph, was a key witness in the murder trial of Larry Cowell, who was convicted in 1982 of having Thompson's nephew pushed from an airplane during a soured drug deal.

Cowell currently is serving a 25-years-to-life prison sentence for Campbell's death. Don DiMascio, who was hired by Cowell to kill Campbell, was convicted last summer of first-degree murder and was sentenced to life imprisonment without parole, Goethals said.

''Knowing the type of people involved in Campbell's murder, retaliation against Thompson is very possible,'' Goethals said. ''Certainly it is an angle that will be explored.''

Friends told police that Thompson said he had received several death threats, but sheriff's investigators said he never reported them.

A racing trade group on Thursday put up a $30,000 reward for information leading to the arrest of those responsible for the Thompson murders.