The Alexandria boyfriend of Margaret F. Yeatman, the Annandale woman whose body was found in the trunk of her car June 29, is being investigated in the death, according to a police affidavit filed in Alexandria Circuit Court yesterday.

Arthur L. Cunn, who allegedly had been having an affair with Yeatman since 1980, led a "double life" according to the affidavit. It said that while working at the Treasury Department and dating 46-year-old Yeatman, a security specialist at the State Department, Cunn was married to a woman in New York.

She moved to Virginia the day after Yeatman's body was found in her car, parked at Johns Hopkins Hospital in Baltimore, according to the affidavit. It was filed in the court to support a request for a search of Cunn's home.

No arrests have been made in the case and Cunn could not be reached for comment yesterday.

Cunn, who lives at 432 N. Armistead St., became involved in the investigation after police learned he began wearing a cast on his right hand June 23. That was the day Yeatman was reported missing, the affidavit said.

Police also found scratches on Cunn's hand and said he told them he owned a .22-caliber semiautomatic handgun, the court paper said. The chief medical examiner for Maryland said fragments from a .22-caliber weapon were found in Yeatman's brain.

The examiner said Yeatman had been shot in the back of the head probably five days before her severely decomposed body was found in the truck of her Toyota Corolla.

In searching Cunn's Alexandria apartment, near the Landmark Shopping Center, police said they found some of Yeatman's papers and personal belongings.

On July 3, Cunn's desk at the Treasury Deparment was also searched, and according the affidavit signed by Alexandria police Detective Anthony Harper, a "pair of glasses were lying on top of the desk identical to a pair found under the body of the victim."