3/17 -- Law enforcement sources tell TMZ ... cops are investigating whether Cali's killing was over a family dispute -- namely, whether Cali blocked Comello from dating his niece. We're told police believe this is something that could've possibly motivated Comello to shoot Cali.

2:21 PM PT -- The NYPD held a press conference Saturday, saying Anthony Comello is in custody and expected to be charged for Frank Cali's murder. NYPD Chief of Detectives Dermot Shea told the media that they believe Comello is the shooter. He's currently being held in a New Jersey jail but will be charged in Staten Island. No word on an official motive.

Police have apprehended a person of interest in the murder of Gambino crime boss, Frank Cali ... TMZ has confirmed.

Law enforcement sources tell us cops picked up a white 24-year-old man in New Jersey Saturday morning, who is now being questioned in connection with the killing.

As we reported ... someone rolled up to Cali's Staten Island home Wednesday night and opened fire on the mob boss, apparently running him over as well.

Our sources say the man was apprehended in Jersey in the vicinity of a blue pickup truck -- which is significant to the investigation. According to investigators, witnesses say they saw a man in a blue pickup truck slam into Cali's parked car, prompting him to come outside.

Police say about a minute after Cali confronted the man who'd struck his car, the other guy pulled out a handgun and fired 12 shots ... 6 of which hit Cali. Cops say Cali then tried to crawl under the back end of the other guy's truck before the man drove away.

We're told the 24-year-old, who has yet to be identified, has not been arrested or charged with a crime as of yet. He's simply someone cops wanted to talk to. Our sources also tell us the dude -- whom we're being told is from Staten Island -- was tracked down in part through his cell phone.

Also interesting ... we're told cops don't believe the murder was actually mob-related, as it appears to be more of a personal dispute than anything else.

Cali reportedly took over as head of the Gambino family in 2015. John Gotti was the "boss" before going down for murder and racketeering in 1992.