MS-13 has emerged as the country’s most brutal street gang, and its reign of terror is spilling into New York.

In April, the bodies of four men ages 16 to 20 were found in Central Islip , a working-class community of strip malls and faded clapboard houses at the center of Long Island. They had been beaten with baseball bats and hacked with machetes. Their bodies were dumped in a park.

“This was one of the largest and most brutal mass murders committed in Suffolk County’s history,” Suffolk County Police Commissioner Timothy Sini said in his testimony last month before a Senate homeland security panel.

MS-13, or Mara Salvatrucha, has between 6,000 and 10,000 members in 46 states, according to FBI statistics.

The gang, which takes its name from the Salvadoran peasants who fought in the country’s 1980-1992 civil war, emerged in impoverished Los Angeles neighborhoods among refugees fleeing the conflict. At the time, their main purpose was to protect a nascent Salvadoran community against more established African-American and Mexican gangs, law enforcement experts say.

Since the late 1980s, the gang has spread to Central American communities across the US. Its members are involved in drugs and sex trafficking. Some are affiliated with Mexican drug cartels.

Pockets of MS-13 operate in Brooklyn and Queens. But the gang is most active among Latino immigrant enclaves in Suffolk County, where more than 400 members are believed to operate.

“MS-13 is unlike any street gang that we have dealt with before,” said Guardian Angels founder Curtis Sliwa, who is focusing his group’s anti-crime efforts on down-at-the-heel communities such as Central Islip, Brentwood and Riverhead. “They are organized and behave like a paramilitary organization. And the violence is incredibly brutal.”

The Central Islip massacre came seven months after the murders of two teen girls in Brentwood. Both were beaten to death in an ambush that involved MS-13 members, Suffolk police say.