A veteran New York City firefighter suffered a fatal head injury after falling 20 feet inside his firehouse this week while preparing for a training exercise, authorities announced Friday.

William P. Moon II was preparing for a drill inside his Brooklyn firehouse Monday when he fell, Fire Commissioner Laura Kavanagh said.

After it became clear Moon would not survive, his family “made the difficult decision to donate his organs to save the lives of others,” said Mayor Eric Adams, who added that the donation of organs so that others might live makes Moon “a hero twice over.”

Moon, 47, had a wife and two children.

The fatal fall occurred as Moon was preparing for a training exercise at the firehouse, Rescue Company 2 in Brooklyn, said Chief of Department John Hodgens, who joined Adams, Kavanagh and other officials at a City Hall news conference.

Hodgens said Moon joined the department in May 2002 and spent nearly 20 years at a ladder company in Queens before transferring last January to Rescue 2, an elite unit that doesn’t just fight fires but also rescues window washers dangling from scaffolding and construction workers trapped in collapsed buildings.

“This type of work requires extensive training,” Hodgens said. “Billy was preparing for one of these training exercises when this tragic accident occurred.”

Officials did not release further details about the fall.

In addition to his service with New York City’s fire department, Moon was a volunteer firefighter in Islip on Long Island, where he lived, and served as chief of that department in 2017, officials said.

Tests determined Moon had no brain activity, officials said, but his body remained hooked up to a ventilator Friday to make organ donation possible.

In a statement read by Kavanagh, Moon’s wife, Kristina, said that her husband “always talked about how important it was to help others.”

“We are eternally grateful for the Christmas miracles that Billy will now be able to give others,” Kristina Moon said.

(AP)