“This is just the beginning,” Justin S. Meyers, an assistant commissioner with the Police Department, said in an interview on Friday. “We’re still continuing to build cases.”

Of those arrested, five are in federal custody, Mr. Meyers said, and they are being charged under the RICO organized crime act.

On Monday, the police found skeletal remains in the woods near the Pilgrim Psychiatric Center, which they identified on Thursday as belonging to Jose Pena-Hernandez, 18. They uncovered the bones where they had found the remains of two other teenagers, Oscar Acosta, 19, and Miguel Garcia-Moran, 15, in September.

Like them, Mr. Pena-Hernandez had also been missing for several months before officials identified his body. But unlike those two, both of whom were recent immigrants, Mr. Pena-Hernandez was identified as being a member of MS-13. The police said they located the remains as a result of the investigation.

“Although it’s very disturbing to find these skeletal remains, it is part of the process,” Mr. Meyers said. “We are eradicating gangs from a community. We put pressure on them, we gather intelligence, we find out past crimes. The goal is not only to quell the violence now, but it’s also to uncover past information to build cases on people.”

Last week, another Brentwood resident, Dewann Stacks, 34, was found beaten to death on a street. The police believe he, too, was killed by members of MS-13.

The most recent spate of violence associated with MS-13 ignited in mid-September, when two best friends, Nisa Mickens, 15, and Kayla Cuevas, 16, were found dead 50 feet from each other in a cul-de-sac close to an elementary school. Their bodies were badly beaten, and the police have not determined a motive.

A friend of Kayla’s mother, Evelyn Rodriguez, said Ms. Rodriguez did not want to comment on the recent arrests.

MS-13 was formed in Los Angeles in the 1980s by immigrants from El Salvador escaping civil war. The gang began to expand to other cities, and back in El Salvador when some of its members were deported.

The authorities say the gang has been in Suffolk County since around 1998, and is organized in cliques bearing names like the Brentwood Locos Salvatruchas. Since 2009, MS-13 has been accused of at least 16 murders, court and police records show.

Timothy Sini, the Suffolk police commissioner, said in a news conference on Thursday that the arrests, conducted over the last month, were the result of increased patrols and the cooperation of multiple law enforcement agencies. He addressed the people of Brentwood, warning them not to be alarmed by the sound of helicopters or heavier police presence around schools.

“We are in this for the long haul,” Mr. Sini said.