The deported MS-13 gang member who was caught back in Long Island this week snuck into the US over the southern border — and may have had help from ganbangers in Suffolk County, law enforcement sources told The Post Friday.

William Umberto Martinez Chavez, 40, was first kicked out of the country back in 2017 for a fatal 2000 stabbing — but was found in Huntington on Tuesday morning and arrested by Immigration and Custom Enforcement agents for illegal reentry.

Investigators don’t think he crossed into the States at an official port of entry — because he was not interviewed by border patrol officers and would’ve been denied entry if he had been due to his previous deportation, sources say.

Gang members who are kicked out of the country and make it back into the US typically pull it off with help from others in the crew who are still stateside, sources said.

Once the gang member is ousted, they must report back to the leaders of the group in their home country and show some proof they were deported, one source explained.

Those who want to return back to the US then typically enlist the help of fellow members in America, who’ll pitch in and gather the money needed to pay a coyote to get their gang brother back into the country.

Without a deportation order hanging over their head, it’s otherwise surprisingly easy for a gangbanger to get into the US themselves, a federal source noted.

Being a possible gang member isn’t enough to deport someone or deny them entry to the US, sources said.

Even someone with MS-13 tattooed on his face could still be allowed to stay in the country for immigration hearings if they claimed asylum or were busted illegally crossing the border, a source added.

Martinez Chavez has MS-13 ink on both his chest and stomach — although claims he only joined the gang in prison for protection, according to Newsday.

In 2018, ICE agents arrested more than 10,000 gang members in both criminal and administrative busts — including more than 2,000 MS-13 members, according to the agency.

The FBI estimates there are over 10,000 MS-13 members nationwide, with at least 1,000 across Long Island.

The group — whose motto is “murder, rape, control” — has long held a stronghold on Long Island’s eastern end and recently trickled further west into Nassau County, where authorities say its members were behind a spate of recent murders.