The Bronx district attorney’s office has opened an investigation into the landlord and building where two toddlers died after a radiator in their city-funded shelter apartment exploded on Wednesday.

The two sisters, Scylee Vayoh Ambrose, 1, and Ibanez Ambrose, 2, were burned by steam from the radiator explosion in their family’s apartment at 720 Hunts Point Ave. in Hunts Point on Wednesday. They both died at a hospital later that day.

“No one has ever seen anything like this,” Mayor Bill de Blasio said on Thursday, adding that the explosion appeared to be a “freak accident — a series of painful coincidences that led to the loss of these children.”

Calling the family’s home “warm and loving,” de Blasio vowed a full and thorough investigation. He said exactly what went wrong remains unclear, but that somehow the valve on the radiator came off and steam came out in an “extraordinary quantity.”

Comptroller Scott Stringer said Thursday there are more than 60 open violations issued by the city’s Housing Preservation and Development and the Department of Buildings on the Bronx building.

So far this year, the address has been sited for several Department of Buildings complaints, including elevator violations, prohibited hardware and holes found in the stone foundation, according to the agency. Some of the elevator violations remain open with no compliance filed.

The building’s landlord, Moshe Piller, was named the city’s fourth worst landlord in 2015, according to a list put out by the public advocate’s office. He was removed from this year’s list because he cleared enough violations.

The building is considered a homeless cluster site, defined by Stringer as a building with a mix of renters and Department of Homeless Services clients.

“Currently, cluster sites not previously identified for closure have more than 13,000 open violations — including nearly 1,000 that are ‘high priority’ and especially dangerous,” Stringer said in a statement.

Along with Public Advocate Letitia James, Stringer criticized the city for not following through on a promise to phase out of the cluster program.

“The city has made minimal progress toward that goal,” Stringer said.

James said it is “unforgivable” for the city to renew contracts with owners of this type of building.

“No funds should be provided to landlords to house homeless families unless full floor-to-ceiling, building-wide inspections are conducted and reveal no hazardous conditions,” she said.

De Blasio, however, insisted on Thursday the apartment had no major health or safety violations when it was inspected last month, and nothing specific that would “indicate anything like this” could happen.

And while de Blasio said he wants to get rid of cluster sites, he said it’s not as simple as just shutting them down.

“I don’t want people to say that this happened because it was a cluster site,” he said. “Let’s come to grips with some facts ... more people are becoming homeless. We’re trying everything we know to keep them out of shelters ... but this is a constant flow of people we’re trying to [support] and address.”

Stringer also called on the city to tackle the homeless crisis. “We need a clear, transparent, public plan,” he said.

Following the explosion, the HPD inspected each apartment in the building, according to the mayor, and didn’t find any similar circumstances. Heat has since been restored, though de Blasio said the four other families who were part of the cluster site have been removed from the building.

The NYPD, HPD and DHS are conducting investigations into the deadly incident, de Blasio said.

With Alison Fox