New York City fire marshals said Friday that an overloaded power strip caused an apartment fire in Queens that injured 21 people, including 16 firefighters.

The April 6 blaze at the six-story building, which displaced about 240 residents, was an accident, and smoke alarms activated as they should have, fire marshals said.

“NEVER overload extension cords with multiple items, appliances or power strips,” city fire officials said in a Twitter post. “Remember – extension cords are for temporary use only.”

About 400 firefighters responded to the afternoon blaze that started on the top floor, and it took them about 12 hours to extinguish it.

Be #FDNYSmart, check cords regularly and replace any damaged or frayed cords immediately See more at https://t.co/NpwdRdDfdw pic.twitter.com/xNdZapXstN

— FDNY (@FDNY) April 17, 2021

Two of the injured firefighters were left with burns that were not life-threatening. No deaths were reported.

A resident in the apartment where the fire started left the door open when fleeing, which helped the fire spread, officials said.

(AP)