Authorities responded to the scene at 170 Ave O around 8 p.m. for reports of cardiac arrest, which was actually determined to be carbon monoxide-related.
The call was upgraded to a hazardous material incident, and of the six total patients, two were pronounced dead on the scene.
ALSO READ | Miracle kidney match leads to Connecticut couple's home getting burglarized
The deceased victims, 50-year-old Juan Tzunun and 49-year-old Jacinto Vasquez, were found in a basement apartment.
Both had been living in the basement for years, and it appears the 911 caller, the owner of the building, is a relative of Tzunun.
The other four victims were all treated at the scene but refused to be taken to a hospital.
Umberto Tzunun said his older brother was one of the victims, and that he was like a father figure to him.
"He was amazing," Tzunun said. "He's like my father because I don't have a father here."
Department of Buildings investigators found the boiler chimney was blocked with debris, and there was evidence of gas backup in the boiler flue.
The investigation also determined that the building had been illegally converted with the addition of illegal single room occupancy units, including one in the cellar and three on the upper floors.
There was also illegal unpermitted electrical wiring in the building, officials said, and tenants in the building did have adequate egress in the event of an emergency.
"All New Yorkers deserve a safe place to call home," the DOB said in a statement. "Again, we are sadly reminded of the potentially deadly consequences of illegal cellar conversions and carbon monoxide leaks. DOB inspectors will be back on scene today with our partners in law enforcement, and the investigation into this tragic incident is ongoing."
Fire officials say the lives could have been spared if there were carbon monoxide detectors.
As a result of the investigation, a Cease Use Order has been issued for the boiler and water heater in the building.
Violations were issued to the building owner for illegal work without a permit, failure to provide occupants with appropriate egress, and for the illegally converted single room occupancy units in the building.
In addition, a Full Vacate Order was issued for the entire building.
The American Red Cross is offering emergency relocation assistance to all affected tenants.
Additional enforcement actions are pending the investigation.
ALSO READ | Caught on video: Man randomly punches child waiting for friend in NYC
* More Brooklyn news
* Send us a news tip
* Download the abc7NY app for breaking news alerts
* Follow us on YouTube
Submit a News Tip