From the New York Daily News’ Joe Mahoney.

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

Berkowitz (above) 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.

Chain Gang, unavailable for comment.