“My neighbor Janet, the Fiores, and Lenny, we’re all on one side of the block, we all got them,” she said. “An entire row of houses.”

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.

They were part of a deluge. From September to December, more than 3,000 complaints of illegal conversion were filed in three Queens neighborhoods — Whitestone, Flushing and Malba. That was more complaints than are typically made in the whole city for an entire year. “You had 500 in one two-week period,” said Dan Halloran, the City Council member for the area. “There hadn’t been 500 calls in two years.”

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.