On March 31, riders of a downtown No. 6 subway train creeping toward the Bleecker Street station heard an unsettling announcement.

"Ladies and gentlemen, we are moving slowly because a building has collapsed on Houston Street."

Looks of exasperation were replaced with looks of worry. "Didn't a building come down last week?" a young woman on the train inquired.

Yes, it did. In fact, the three-story building that caused the slowdown, 41 East Houston Street, was part of a spate of recent collapses.

Engineering experts saw some of them coming, and the city, in hindsight, has moved to shore up a system that includes unlicensed demolition work as the norm.

The first collapse, on March 27, sent a construction crew running from a five-story building at 1861 Lexington Avenue in Harlem as it caved in around them. Preliminary findings from the city Department of Buildings indicated that ongoing construction might have compromised the structural integrity of the building.

A week earlier, the Buildings Department stopped demolition work at 41 East Houston after cracks were found in one of the building's bearing walls. Days before a section of the building crumbled, the department issued a violation for failure to safely carry out the demolition; the building was undergoing a conversion into an 11-story condo.

It wasn't until an incident on April 5, however, that New Yorkers really started to take notice.

Contractors were conducting stabilization and repair work inside Chumley's, the popular West Village speakeasy-cum-burger-joint, when a section of the chimney separated from the interior wall and fell into the bar area.

The Chumley's collapse set tongues wagging in the city, and not just because the neighborhood would be without pub fare for the foreseeable future.

Margaret Streicker-Porres, the owner of 86 Bedford, where Chumley's is located, has received a litany of complaints from the Buildings Department over the last year for doing construction without the appropriate permits. After the recent collapse, 86 Bedford was issued three more unsafe-construction violations.

ENGINEERING EXPERTS HAVE BEEN increasingly worried about this issue over the last few years--years that have corresponded with a building boom throughout the five boroughs.

"This has been a big problem for some time," structural engineer Joe Tortorella told The Observer. "It is a consequence of overzealous contractors and owners trying to work too fast and make money, and ultimately cutting corners."

Mr. Tortorella, a former president of the Structural Engineers Association of New York, an engineering trade group, thinks that the incidents stem largely from inexperience.

"You don't have to be licensed for demolition in New York City, so you have guys that do not know what they are doing," Mr. Tortorella said. "When the city is this busy with construction, you wind up with a lot of unsupervised demolition and construction work going on."

New York has indeed been busy. In 2006, the city O.K.'d 30,927 building permits for privately owned housing units; in 2005, that number was 31,450. These were the busiest years since 1972--and that year, as City Hall has pointed out, was busy only because of subsidized-housing construction.

These three collapses are merely the latest. In October 2006, a two-story building at 23 Second Avenue partially collapsed. A few months earlier, a building housing a Gristedes supermarket at 100th and Broadway came down as it was being removed to make way for two condo towers.

When a building collapses, partially or completely, the Buildings Department sends in a forensics unit to figure out what happened (think CSI, without the bodies). Following the inspections of the buildings in the three most recent collapses, the Buildings Department issued violations to two of the trio for unsafe construction practices.

But don't call it a trend quite yet, the city says.

"Most of the construction going on in the city is being done in a safe manner," Kate Lindquist of the Buildings Department told The Observer. "The fact of the matter is there are over 950,000 buildings in this city, and many of them are pushing 200 years old. If they're not cared for, structural issues can ensue."

The Buildings Department is taking steps toward preventing buildings from giving way. The day that 41 East Houston collapsed, the department launched the Low-Rise Site Safety Program, which will conduct random inspections of buildings under construction between seven and 14 stories tall.

The department is also instituting a measure called the Excavation Notification Requirement, which will require contractors to notify the department prior to beginning excavation so that it can monitor the work.

None of the three buildings that collapsed in late March and early April are in the 200-year-old range cited by the Buildings Department as particularly vulnerable to collapse. In fact, 86 Bedford is the oldest and it was built in the late 19th century, according to the Web research engine PropertyShark.com.

Ms. Streicker-Porres, the owner of 86 Bedford, and Focus and Struga Building Development, the contractor working at 1861 Lexington, didn't return calls for this article. Trevor Stahleski, a principal in Cardinal Real Estate Investments, the owner of 41 East Houston Street, maintained that he followed protocol while doing work.

"The building was old and deteriorating and came down on its own," Mr. Stahelski told The Observer. "We worked with the Department of Buildings leading up to the collapse, because we knew it was going to happen."