New York- Son of Sam killer David Berkowitz apparently had a gay jailhouse fling with a fellow serial murderer, according to a veteran homicide cop.

Berkowitz traded a series of flirtatious letters with cross-dressing, muscle-bound killer Gary Evans in the late 1980s while the two were locked up in the same state prison, the investigator says.

The letters surfaced when former state police sleuth Jim Horton and Connecticut writer William Phelps teamed up for a book about the detective’s relentless pursuit of Evans, who took a fatal plunge into the Hudson River in 1998 after kicking his way out of a police van.

In one letter, penned in 1987, Berkowitz wrote: “Gotta lose some more weight so that I could fit into this new bikini I got, a nice two piece that will make all the boys go crazy.”

Evans’ friendship with Berkowitz is detailed in “Every Move You Make,” published by Pinnacle and due out tomorrow.

Horton told the Daily News the Berkowitz letters were later found in a box along with the bisexual Evans’ stash of gay sex magazines.

“Gary acted like a tough guy because he was insecure about his bisexuality, and looking back it’s pretty clear his relationship with Berkowitz was a homosexual one,” said Horton, now deputy chief investigator for the state attorney general’s office.

In another letter from the same period, Berkowitz addressed Evans as “the Tricep King” and suggested he was a 12-year-old boy admiring Evans’ photo in a muscle magazine.

Berkowitz and Evans met while both were confined in maximum-security Clinton Correctional Facility in upstate .Dannemora. Evans was blamed for five murders – four of them after he met Berkowitz.

Berkowitz, 52, is serving 25 years to life for killing six people and wounding seven in a mid-1970s spree in New York City. He became a born-again Christian in jail.

In another development yesterday, Berkowitz filed suit in Manhattan Supreme Court against a former lawyer, saying the attorney tricked him into turning over all of his personal belongings, including hundreds of letters, bar mitzvah photographs and pictures of his late mother.

Berkowitz wants the lawyer, Hugo .Harmatz, to give the items back.

Berkowitz is also fuming that Harmatz wrote a book about him, “Dear David,” which will be on display, along with Berkowitz’s memorabilia, at a book expo tomorrow at the Jacob Javits Convention Center.

Neither Harmatz nor Berkowitz’s current lawyer could be reached for comment.