A gang of "sophisticated" thieves used tools to rip through concrete walls and steal $1.8 million from a Hicksville armored car company but were foiled by an alert Nassau police officer, who arrested one of the suspects, authorities said Wednesday.

Investigators are still looking for at least two other suspects in the heist, and officials Thursday plan to announce a substantial reward for information leading to arrests.

The burglary occurred at the warehouse of Loomis Armored Inc., 74 Alpha Plaza, at 10:20 p.m. on Aug. 16, said Deputy Insp. Gary Shapiro, a police spokesman.

"This was a very sophisticated group that committed this burglary," acting Nassau Police Commissioner Thomas Krumpter said in an interview Wednesday, adding that the vault penetrated by the thieves held roughly $20 million.

"The great job that the officer did in this case can't be understated," he said. "He did a fabulous job."

The Second Precinct officer was on patrol that night when he saw a man in the warehouse parking lot acting "suspicious," Krumpter said.

The officer, whom police declined to name, arrested Edgar Medina, 53, of Brentwood, who was not armed, at the scene. The cash was in the trunk of Medina's car, Krumpter said.

Medina was charged with third-degree burglary, first-degree grand larceny and possession of burglar tools, police said. He's being held in jail in lieu of $300,000 bond or $150,000 cash bail, according to online court records. An attorney for Medina could not be reached for comment.

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"They went through walls. . . . They broke through concrete and several layers of hardened security," Krumpter said, declining to say what kind of tools the burglars used.

Further details about the heist and the suspects being sought were not released.

An employee who answered the phone at Loomis Wednesday night said no one was available to comment.

Nassau County Executive Edward Mangano said in a statement: "I commend the police officer who stopped this elaborate scheme to steal millions of dollars."

Acting District Attorney Madeline Singas said the heist "could be the stuff of TV fiction," and added: "Detectives have been working tirelessly to finish cracking this case of brazen and wanton greed, but we need the public's help to catch those accountable."