Even for Washington, it was a strange get-together. The site was a red brick warehouse in a rundown part of town. The guest list of 60 was a rogues' gallery of burglars, robbers and other menacing types. They wore tuxedos, and 30 of them carried narcotics. One had just broken away from an arresting officer and arrived in handcuffs. Another, a pimp with an eye toward business, brought two accommodating young women with him.

The guests were allowed to enter slowly in groups of four. Once inside, each was frisked by the host, Pasquale "Patty" LaRocca, also in dinner jacket, who then introduced them to the "Don" from New York. "Bless you, my son," the Don mumbled to each crook. At that point, Pasquale stepped in to announce: "I'm a cop. You're under arrest," and a team of officers in riot helmets emerged, handcuffed the four guests and led them to a back room. "Patty," said one puzzled man, "tell me this is some kind of mistake."

It wasn't. And last week, the weird caper—in which LaRocca and five other cops and agents posed as "fences," or receivers of stolen goods—ended up with a bag of 135 suspects. Another 48 were on the lam.

LaRocca was really Patrick J. Lilly, 27, of the Metropolitan Police force. His cohorts were three Metropolitan Police officers, an FBI agent and a Treasury man, all masquerading under names like Angelo Lasagna and Rico Rigatone. (The visiting Don was played by a beefy police sergeant in a one-night stand.) The undercover cops ran a fencing operation in the warehouse for five months where they bought some $2.4 million in stolen merchandise for $67,000. The loot included furs, guns, stereo and TV sets, calculators and even a heart-lung resuscitator.

Inspiration for "The Sting," as it was called in Washington, came from police Lt. Robert Arscott, 40, and FBI agent Robert Lill, 30. Angry over the more than 3,000 typewriters stolen from offices in his precinct, Arscott opened the two-story warehouse last October and put out word that the "New York Mafia" would pay top dollar for hot goods. Criminals turned up in such numbers that they often had to queue up to gain admission.

The star of the troupe, Pat Lilly, is a slim, articulate detective with eight years on the force. He thought of becoming a veterinarian, but chose police work because "I thought maybe I could help people." To prepare for his role, Lilly dyed his sandy hair black, grew a beard and explained away his blue eyes by attributing them to an "Irish mother." He wore an electric blue suit and open-collared shirts, and was so convincing that he persuaded one hood who fingered him as a cop that it was a case of mistaken identity.

Some of the criminals said they were interested in jobs as hit men with the mob, and bragged of murders they had committed—all of this into a camera and microphone hidden in a wall. At times Pat had to tell them to speak up because, he explained, his hearing had been damaged in a shootout.

Despite some grimly funny aspects, the assignment was dangerous and unpleasant. The warehouse was frigid, and because the cops were often working 15-hour shifts, they all caught colds. Once a thief tried to hold them up, but Lilly and his pals drove him off with a shotgun. Pat kept in touch with his family mostly by telephone; sometimes he did not go to his suburban Maryland home for days at a time.

At the end of five months the cops decided to move since they had run out of purchase money and Lilly had the names, addresses and phone numbers of most of his shady customers. "Come to my party," Pat told them, "you're sure to get some good times."

And so they came and were hauled away. Lilly is now back on more routine assignments, mulling over offers for book and film rights to his story. He regrets that the operation had to be closed down so soon. "We put most of the fences out of business," he says. "If we'd gone on a little longer, we'd have gotten them all."