An expert says MS-13 is escalating the level of brutality in their attacks.

MS-13 is believed to be responsible for 17 murders in Suffolk in the last year and a half - 11 in 2016 and six so far this year. In March, authorities arrested 13 gang members -- a roundup that included the alleged killers of Brentwood teens Kayla Cuevas and Nisa Mickens.

Retired Suffolk Detective John Oliva was the top MS-13 expert on the FBI's Long Island Gang Task Force. He says the gang, which has been on Long Island since the late 1990s, is escalating the level of their brutality - including choosing to use machetes over guns.

“Now you're actually getting close and personal with your victim,” says Oliva. “I mean you're right on top of them ... you're actually stabbing them, you're hacking at them, the blood has to be squirting out.”

Oliva also says the gangs in El Salvador are more violent than here, but that gang members coming from El Salvador to the U.S. are bringing that mentality with them.

“Dave” is the father of a Brentwood teenager who says he was assaulted by gang members at his school. Dave first talked to News 12 seven months ago, and he says he and his sons, as well as other teens, continue to live in fear. He is now finalizing plans to move his family out of state.

No one has been arrested in the killings of the four young men a month ago in Central Islip. Police have offered a $25,000 reward for anyone who provides information about the crime.