A savage brawl between two warring groups of FDNY firefighters at a Bronx firehouse has sparked multiple investigations and shocked even hardened department brass, The Post has learned.

More than a dozen members pummeled three rivals with punches and kicks to the head during the melee, sources said.

“It was one of the most severe beatings they’ve seen on tape,” an FDNY insider said, recalling the reaction by brass who viewed video of the bloody fisticuffs.

“They can’t believe one of the guys wasn’t killed.”

The tape shows Adam Soler, 35, of Engine 68, being held down by two firefighters from Rescue Co. 3 and repeatedly kicked by a third, said sources who were told about the video.

“Someone grabbed his head and started pounding it into the pavement,” one said.

One firefighter accidentally kicked a curb. He is seen grimacing and holding his foot, which may have been broken, a source said.

Soler suffered a concussion and has been on medical leave since the June 6 fracas.

Capt. Michael Nigro of Engine 68 became so upset watching a clip of his firefighters being beaten, he ordered staffers to briefly pause it.

“He went outside to compose himself before coming back in to watch the rest of it,” an insider said.

Yet two months after the June 6 melee, no one has been arrested or fired.

In an internal order, the department has since identified six participants in addition to Soler. All were suspended for one month but are now back on the payroll.

“If they assaulted anyone, they shouldn’t be getting a paycheck,” an FDNY insider fumed, blaming Fire Commissioner Daniel Nigro for failing to take strong action.

“They should be getting a prison cell.”

The fracas marked another black eye for a department that has been plagued by violent clashes among its ranks.

The brawl recalled a 2003 New Year’s Eve attack at a Staten Island firehouse, nicknamed “Southern Comfort,” when Staten Island firefighter Robert Walsh was badly beaten by a chair-wielding colleague during a booze-filled celebration.

The June 6 battle came after FDNY members from upper Manhattan and the Bronx traded insults and skirmished earlier that day at Billy’s, a bar near Yankee Stadium, following an awards ceremony.

Instagram videos showed firefighters tussling with cops as police tried to break things up.

One poster wrote: “NYPD VS NYFD a group of drunk firefighters fighting with pedestrians and NYPD around Yankee stadium @ 940pm. The police did nothing but send them home.”

Inside Billy’s, a clash erupted when a firefighter from Ladder 44 tossed a commemorative T-shirt from Engine 68/Ladder 49 into the trash, a source said.

Then a Rescue 3 captain allegedly hurled “the N-bomb” at an Engine 68 firefighter who is black, sources said.

Hours later, around 10 p.m., Soler and his brother-in-law William Fitzpatrick and James McGowan, both of Ladder 49, showed up at Rescue 3 on Washington Avenue and complained about being disrespected at Billy’s.

At that point, a group of 15 to 18 firefighters, not all assigned to Rescue 3, ganged up on the three visitors, punching and kicking them, said those told about the video.

The strapping Soler was held down and pummeled, they said.

The NYPD confirmed it made no arrests and filed no reports on the clash at Billy’s, but a spokesman did not respond to questions last week about its response to the fight at Rescue 3.

On June 9, the FDNY issued a statement saying the department had “obtained disturbing video,” which sources said included footage from both fights that day, and turned over tapes to city investigators.

“Everyone and anyone who engaged in this outrageous behavior will be held accountable,” Commissioner Nigro said.

Soler declined to comment last week, referring questions to his attorneys, who did not return messages.

Fitzpatrick, also suspended for 30 days, would not comment last week. McGowan, who was not suspended, did not return a message.

An FDNY internal order listed the other suspended firefighters as Lt. Glen Berube, Duane Davis, Daniel Foley and Joseph Killeen, all of Rescue 3, and Lawrence Munafo Jr. of Engine 68, and Michael Pattwell of Ladder 44.

All have returned to work, except for Soler, but remain on restricted duty pending an ongoing investigation, insiders said.

FDNY spokesman Jim Long refused to answer any questions, citing the “active investigation.”

The city’s Department of Investigation and the Bronx DA’s Office are probing the matter.