By flooding the city’s 311 line with phony building complaints, someone has put thousands of homeowners in Queens under a state of bureaucratic siege.

And while no one has been identified with certainty as the source of the calls, citizen investigators have pieced together enough scraps of information to draw the outline of a money-making racket by home-improvement scamsters.

On the morning of the last big snowstorm in February, Joseph Choi spotted two city building inspectors on his block, 156th Street in Flushing, talking to a neighbor.

“Everyone thought they were coming about shoveling the sidewalk, and it was still coming down hard,” Mr. Choi said. But it turned out the inspectors were checking a report made to the 311 line that the owners of the house had illegally turned a basement into an apartment.

The following week, the inspectors were back, knocking on more doors, rattling more nerves.

To help an elderly neighbor and another one who does not speak English fluently, Mr. Choi, 35, got on the Department of Buildings Web site to see if any problems were listed with their homes.

“Both of them had the same complaint against them: illegal conversion of a basement or first floor,” Mr. Choi said. “Then I scanned every address on the block. Every single house had the same complaint.”

Including his?

“My house, too — it was ‘illegal conversion of basement,’ ” Mr. Choi said. “I don’t even have a basement.”

One block away, on 157th Street, a similar discovery was made by Vana Partridge, who was also visited by an inspector during the snowstorm.

“I asked if he had a warrant, he said no, and I said he’s not allowed in,” Ms. Partridge said. Before long, she and her neighbors all found that someone had filed complaints against them for illegal conversion.

In fact, whole neighborhoods were targets. “I found 65 complaints in a 10-block area,” said Sandi Viviani, president of the Broadway Flushing Homeowners Association.

The complaints were routed to the Department of Buildings, which dispatched inspectors to check out the homes.

But who was making the complaints?

An obvious question, but no easy answer. “You can’t just take a complaint and tie it back to a phone number,” said Nicholas Sbordone, a spokesman for the Department of Information Technology and Telecommunications, the city agency that runs 311. “We encourage people to file anonymously. They can call and choose not to leave their phone numbers.”

While the phone numbers of callers are displayed at the 311 center when people dial in, the numbers are not automatically entered into complaint forms that the call-takers fill out, Mr. Sbordone said.

There are a few tantalizing clues. Actually, a thousand of them. During the three months that the building complaints were spiking in Queens, the city received its typical five million calls at 311 on all kinds of issues. “We found that more than 1,000 came from three very closely related numbers, that all appeared to be from one business,” Mr. Sbordone said.

The matter is now being investigated by the city’s Department of Investigation and the Queens district attorney’s office.

The homeowners in Queens have a few suggestions.

Right around the time that the complaints and inspections were landing, people in Whitestone, Flushing and Malba began to find fliers from several different companies in their mailboxes, or stickers on newspapers at their doors, according to Dennis Ring, the chief of staff for Mr. Halloran.

“The fliers said things like, ‘Received a violation? Get a free consultation,’ ” Mr. Ring said. And, of course, even though few people had illegal conversions, once the inspectors got in the door, they were able to find some violation or another of the code.

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