NORWALK, Conn. – A retired police chief testified Wednesday that he unsuccessfully sought an arrest warrant for the older brother of Kennedy cousin Michael Skakel in the 1975 murder of a 15-year-old girl.

Thomas Keegan, retired from the Greenwich force, said he presented an affidavit in 1976 to the prosecutor’s office seeking the arrest of Thomas Skakel on murder charges in the beating death of Martha Moxley.

Prosecutors rejected the request, saying there was not enough evidence to establish probable cause, said Keegan, who was a police captain at the time and a lead investigator in the case.

Moxley was beaten to death with a golf club on Oct. 30, 1975. Michael Skakel was also 15 at the time. Thomas Skakel was 17. The brothers are nephews of Robert F. Kennedy’s widow, Ethel.

Now 41, Michael Skakel was indicted in January 2000 by a judge sitting as a one-man grand jury. If convicted, he could face life in prison.

Defense attorney Michael Sherman asked Keegan if he believed there was enough evidence to charge Thomas Skakel with the crime.

“You wouldn’t just sign [the warrant] willy-nilly?” Sherman asked.

“It would not be a frivolous thing,” Keegan responded.

Thomas Skakel was an early focus of the investigation along with Kenneth Littleton, a tutor at the Skakel home. Authorities are basing the case against Michael Skakel on admissions he allegedly made years after the murder.

Keegan said he never applied for an arrest warrant for Littleton and had no idea what has happened in the investigation since he retired in 1986.

The testimony was allowed over the objections of prosecutor Jonathan Benedict, who said prosecutors have filed a motion seeking to limit the ability of the defense to suggest a third party committed the murder.

Benedict downplayed the warrant request. He said it simply indicated that at some point at least one police officer thought there was evidence against Thomas Skakel, who was the last person seen with Moxley.

Also Wednesday, jurors passed around a shoe Moxley was wearing the night of the murder. The word “Tom” was written on the shoe.

“That name was present when the shoe was taken from the victim,” Keegan testified.

The case went unsolved for years, leading to suspicions that wealth, privilege and the Kennedy connection somehow had protected Skakel. The slaying regained attention after several books were written about it.

Moxley’s body was discovered beneath a tree on the family’s Greenwich property. She had been beaten with a golf club that investigators matched to a set in the Skakel household.