Property owners often illegally convert rooms and buildings into spaces that are presented to renters as apartments but do not meet city building code requirements.
Mayor Michael Bloomberg's administration on Thursday detailed the sting operation it began last spring to crack down on the practice, which officials said is common in this city where rents are sky high. More than 100 violations were found in 62 apartments inspected.
Bloomberg said there are estimates that "many many thousands" of people live in illegally converted spaces.
In the case of illegal apartments, the worst case scenario was found in a building in Bensonhurst where five people were killed.
In thousands of other cases, renters are put at great risk.
"This landlord is going to show us the attic apartment," said Robert LiMandri, the New York City Buildings Commissioner.
Using undercover cameras, Buildings Department investigators posing as renters and gaining access to apartments, discover illegal living conditions in 54 of 62 places inspected.
"You will see the illegal plumbing work. This is the shower in the bathroom. You can see extension cords. The electrical wiring is illegally installed here," LiMandri said.
Officials say 33 properties were so dangerous they issued orders to vacate.
In one basement apartment for $750, utilities included, in addition to illegal plumbing and a gas stove, investigators made one of the most shocking discoveries.
"See this large silver pipe? That's actually the flu pipe from the boiler running right through the apartment. We all know Carbon Monoxide kills," LiMandri said.
And so do other conditions.
The illegal apartments, as firefighters discovered 5 years ago in the Bronx, become a deadly maze of walls and doors.
"We had a fire officer and a young firefighter jump to their deaths in the Bronx following an illegal conversion," Richard Tobin, the Assistant Chief of Fire Prevention said.
The consequences for renters can be just as deadly.
Five people were killed in Bensonhurst earlier this year.
"The only means of escape was the stairwell where the fire had started, so they couldn't get out. The bottom line is that illegal conversions can kill," Mayor Bloomberg said.
Just like many innocent renters do, investigators poured through apartment listings on Craigslist, looking suspect conversions.
They say apartments must always have two ways to exit.
They must have smoke and carbon monoxide detectors with working batteries, and an extension cord should not be used as a main source of electricity.
A recent Eyewitness News investigation turned up even more of what officials estimate are thousands of illegal conversions that were often difficult for investigators to gain access to.
"We're not just going to knock on doors. We're going to do other things. We're going to hold them responsible," LiMandri said.
More than 100 violations were issued, with penalties ranging from $6,000 to $25,000.
To resolve the violations, property owners must correct the illegal construction work and submit certificates of correction or sworn affidavits to the city explaining how the problems were fixed.
(The Associated Press contributed to this report)