Mayor Bill de Blasio has vowed for a thorough investigation into a radiator explosion in a Bronx shelter that killed two toddlers Wednesday afternoon.

Firefighters found 1-year-old Scylee Ambrose and 2-year old Ibanez Ambrose with severe burns at their building on Hunts Point Avenue around noon.

They were rushed to Lincoln Hospital, where they were pronounced dead.

The victims and their parents lived in one of the five units in the building used as a shelter by the city's department of social services. It is a cluster site, which is a landlord-operated homeless shelter.

The radiator was taken out of the shelter Wednesday night to be examined, and the city searched the building for clues.

The last time the department of homeless services inspected the apartment was Monday.

The mayor says the radiator's valve appears to be the root of the incident.

"What we know so far suggests an extraordinary and unprecedented accident, something that we, no one I've talked to so far in any agency has ever seen anything like this," de Blasio said. "This was a freak accident, a series of painful confidences that led to the loss of these children."

City records show several complaints on file for the building, mostly for elevator and plumbing problems.

The other four families housed by Social Services are being moved out.

The last building complaint that city officials had for the building was from April for a broken radiator in a different apartment. Officials say it was resolved.

The tragedy has been a brutal reminder of the horrible conditions in these types of homeless shelters.

In January, the administration promised to move families out of these buildings.

Only 453 units have closed so far, while 3,205 remain.

De Blasio administration officials hope to end the program in 2018.

The nonprofit provider, known as BEDCO, who placed the family in this building, is responsible for 422 units across the city. City officials say those units have 2,274 violations.

63 of those violations were at the little girls' building.

The landlord of that particular building is known as one of the worst landlords in the city, leaving officials outside of city hall to ask why the Ambrose family was ever put there in the first place.

"This was not a freak accident by no means," Public Advocate Letitia James said. "What we really need to do is to get to the bottom of this and find out who failed to protect these two children."

City officials say shutting down this program will take time. Some 12,000 men, women, and children live in cluster sites across the city. If the city were to shut this program down today, those families would have nowhere to go.