In a terrifying new development, police in Aruba believe that GARY GIORDANO – their prime suspect in the disappearance of blonde beauty ROBYN GARDNER – may have had an American accomplice.

The ENQUIRER has learned exclusively that authorities are hunting for a heavily tattooed man they believe assisted Giordano in murder­ing the dental assistant from Maryland.

The mystery man was seen with Robyn and Giordano at the Rum Reef Bar & Grill just before Giordano claims the pair went swimming and Robyn, caught by a current, never returned, said a source close to the investigation.

“Giordano took out a $1.5 million life insurance policy on Robyn and tried to collect on it just two days after she disappeared. Police believe he may have hired an accomplice to help him kill the unsuspecting woman and share in the money.”

Robyn went miss­ing Aug. 2, but the police only recently learned of the suspect­ed accomplice when witnesses provided first-hand descriptions of the mystery man. Their reports triggered an island-wide man­hunt involving local police and the FBI.

The ENQUIRER, in an exclusive interview with one of the witnesses, has learned that au­thorities are looking for a white American with a stocky but muscular build. He’s about 40, with dark, curly hair and a large tattoo of a heart over a cross on his left arm and another unspecified tattoo on his left hand.

Spotted with Robyn shortly before her disappearance, the tattooed American was wear­ing new Nike tennis shoes, red and white socks, a light-colored muscle shirt and pricey sunglasses.

The witness told police that Giordano, 50, and Robyn, 35, arrived at the Rum Reef restaurant mid-afternoon and ate. The mystery man arrived sometime later.

After a while, Gior­dano left for nearby Nanki beach, while the other American and Robyn talked for another 10 min­utes.

Investigators believe during that two-hour stretch, Giordano and his accomplice drove Robyn to an area of dingy shacks near an oil refinery – where she was killed. They think the two men then buried the body in a shallow grave.

Police theorize that the original plan was for the mystery man to return to the site after Giordano had left the island, recover the body and dump it at sea.

Police suspect the man and Giordano knew each other from the U.S., perhaps meeting on one of the “swinging singles” websites that Giordano often frequented – and that they hatched a plan to kill Robyn for the insurance money.

ALL the grim details are in the new ENQUIRER – on sale now.