The city's Department of Investigation and NYPD have launched simultaneous investigations into a bar fight last week that involved some of New York's Bravest.

Sources tell NY1 members of Brooklyn's Engine 310 Ladder 174 -- who were all off duty -- began their night last Friday at a Park Slope restaurant before heading to a Bay Ridge watering hole, the Salty Dog.

Sources say trouble began when a firefighter spilled a drink on one of the bar's patron's who reacted by punching him in the face.

Several other firefighters and other bar patrons got involved.

NY1 is also told at first, some of them may have been trying to break up the fight, but more fighting quickly broke out.

Sources say the brawl lasted just a couple of minutes, until security broke it up. The entire melee was caught on a surveillance video and is now being reviewed by the NYPD and DOI.

A 911 call was not placed, but police officers from the 68th precinct did respond. Later in the day, four men -- two from Brooklyn and two from Staten Island -- filed complaints saying they'd been assaulted.

Investigators are now looking into who might have been involved.

Fire Commissioner Salvatore Cassano's nephew works in the same firehouse, though sources say he was not in the bar and was not involved in the incident.

The FDNY says since no formal charges have been brought against any of its members, it is not conducting its own investigation.

The department says it is cooperating with the NYPD and DOI as they look into the incident.