CENTRAL ISLIP, N.Y. — The trap was set. The targets were in place. On the night of April 11, the authorities said, more than a dozen MS-13 gang members, wielding machetes, wooden clubs and knives, ambushed a group of young Latino men in a Long Island park, beating and killing four.

A little more than three months later, the police said they had caught the killers.

On Wednesday, the Suffolk County Police Department, after a news conference with federal prosecutors, said that it had made more than 15 arrests of MS-13 gang members in the quadruple homicide and other crimes, including a January homicide at a deli that left another person wounded.

The arrests included four detailed in federal court documents released earlier in the week, when the authorities said they had charged four people in the April killings: Alexis Hernandez, 20; Santos Leonel Ortiz-Flores, 19; Omar Antonio Villalta, 22, known by a nickname, “Anticristo”; and a juvenile whose name was withheld.

The Suffolk County police commissioner, Timothy Sini, said on Wednesday that those arrested in what he called “one of the worst crimes committed in Suffolk County history” included six juveniles.