Mayor Bill de Blasio says New York City is conducting a “rigorous” investigation into a “freak” radiator accident that led to the deaths of two toddler sisters.

De Blasio said Thursday that a “pained silence” filled the air when officials visited the Bronx apartment where 2-year-old Ibanez Ambrose and her 1-year-old sister, Scylee, were fatally injured. He says the officials were surrounded by signs of a “warm and loving household.”

De Blasio says something happened with a radiator valve Wednesday, causing an “extraordinary” amount of steam to come out.

He says the apartment was inspected last month.

The privately owned building was used to house homeless people. But the mayor said there’s no indication that problems with the program were related to the children’s deaths.

(AP)