New York City Mayor Bill de Blasio says it is “not acceptable” that ambulances took more than 10 minutes to respond to fire that killed two 4-year-olds.

The blaze ignited Saturday at a home in Far Rockaway, Queens. Authorities say it was set by the children playing with fire.

Fire Commissioner Salvatore Cassano says an investigation has been launched into why it took an ambulance so long to arrive. The mayor said Tuesday he expects a quick resolution to the inquiry.

De Blasio also says the city’s 911 system must be monitored constantly. In his previous role as public advocate, he frequently expressed concern about delays within the dispatching system.

Cassano is a holdover from the previous administration. He is serving on an interim basis and will be replaced.

(AP)