FDNY fire marshals have determined that Friday's fatal fire in Harlem was caused by a lithium-ion battery.

The FDNY made the announcement Saturday on X, formerly known as Twitter, a day after one person was killed and at least 17 others were injured in a fire on St. Nicholas Place.

The fire started just before 2:15 p.m. on the third floor of a six-story residential building between West 148th Street and West 150th Street, the FDNY said.

Police said the person who died was a 27-year-old man who was found trapped inside the building and suffered from smoke inhalation.

All 18 people treated by first responders were taken to nearby hospitals. At the time, four of them were in critical condition.

The American Red Cross has opened a reception center at PS/IS 210 in Manhattan, where impacted families can register for emergency assistance and get blankets and hot meals. The agency added that it will also remain on scene to support impacted families.

FDNY Chief of Department John Hodgens said at a press conference Friday that rescuing patients from the fire was a "challenge," as there were "heavy smoke conditions" in the upper floors of the building.

The Department of Buildings issued a full vacate order of the building due to damage caused by the fire.

Windows were still broken at the building Sunday, offering a glimpse into just how dangerous the fire was.

Residents were trapped on the top floors, and firefighters used rope to rescue people from their windows, which is a rare event.

Friday marked the first lithium-ion battery fire death of the year in the city, the FDNY said.

In 2023, law enforcement officials carried out 268 investigations related to these batteries, and fires caused by the batteries resulted in 150 injuries and 18 deaths, according to the FDNY.

As of Feb. 19 of this year, there were 24 lithium-ion battery investigations carried out in 2024, the FDNY said.

New Yorkers said they've become extra vigilant when it comes to spotting lithium-ion batteries.

"Where I'm at, if I saw someone with a lithium-ion battery, basically I would call 311. It’s sad, it’s sad,” said Harlem resident Shawn McKeever.

"With all devices, [there are] safety precautions. You got to read instructions to know the proper way to utilize these materials. Do it for safety, not just yourself, but everyone around you," said Gregory Solomon, also of Harlem.

A crackdown on illegal batteries is happening in the city.

Earlier this month, the NYPD and the Queens district attorney’s office teamed up to seize illegal motorized scooters from a scooter shop that was manufacturing and selling illegal lithium-ion batteries.

The FDNY is pushing for lawmakers in Washington to pass legislation to regulate lithium-ion batteries because of how unpredictable and catastrophic they say these fires can be.