The guard on duty during an $11 million robbery Dec. 12 at a Bronx armored car company was arrested yesterday on charges of participating in the theft.

The FBI announced in New York that 24-year-old Christos Potamitis of Queens, a $4-an-hour guard at the Sentry Armored Courier Corp., was arrested while sunning at a hotel pool in Puerto Rico, where he was vacationing.

Also arrested was George Legakis, 21, of Brooklyn, a part-time cook and a friend of Potamitis. Legakis was arrested Tuesday night at the FBI's New York headquarters after he failed a lie detector test.

"We spoiled Potamitis' vacation," said Lee Laster, the FBI's assistant director for the New York region, in a news conference. At least two other suspects are being sought, he said. None of the money, in small bills, has been recovered.

Laster would not describe the evidence against the two men, but James Sullivan, New York City's chief of detectives, told reporters that authorities had a witness. He gave no other details.

Potamitis had claimed that he was alone on duty at Sentry's headquarters in the Bronx when two masked men punched a two-foot-wide hole through the steel and tarpaper roof, disarmed him and handcuffed him to a stairway railing.

He said the men used a crowbar to force their way into the firm's vault-like money room, and fled with $11 million after scrawling "Robbers Was Here" on a dusty mirror.

Authorities said then that they believed at least three men were involved, and they speculated that the robbery was an inside job because of the extensive knowledge the robbers had of Sentry's operations. The thieves reportedly disconnected a silent alarm system.

Police said Potamitis had failed a polygraph test and has been under surveillance since the robbery.

Sentry went out of business two weeks ago, following the disclosure of several other losses at the company.

About three weeks after the robbery, $225,000 in Yonkers Raceway receipts were discovered missing from Sentry's warehouse. And police also discovered that an additional $100,000 had been stolen from the company in June.

Three Sentry officials, including founder and President John Jennings, were arrested and charged in the alleged "skimming" operation.

Asked yesterday whether the three executives were implicated in the $11 million robbery, Sullivan said: "We are dealing with two separate cases."

On Jan. 18, Bronx District Attorney Mario Merola said that at least $500,000 had disappeared from Sentry before the Dec. 12 robbery.

Potamitis and Legakis were charged with bank larceny rather than robbery, authorities said, because robbery implies use of force and larceny only implies complicity. Legakis was being held on $1.5 million bail, and Potamitis' bail was set at $5 million.