A teenager in New Jersey was taken into police custody after his parents, sister and a family acquaintance were found dead inside the family’s home late on Sunday, the Monmouth County prosecutor said in a statement on Monday.

The statement said that it “appears” that the teenager, 16, had used a semiautomatic rifle to kill his father, Steven Kologi, 44; mother, Linda Kologi, 42; sister, Brittany Kologi, 18; and an acquaintance, Mary Schultz, 70, at 11:43 p.m., the prosecutor, Christopher J. Gramiccioni, said.

The police went to the home in Long Branch, a seaside New Jersey town of about 30,000 people, after a 911 call reporting a shooting. The Associated Press quoted Mr. Gramiccioni as saying that the teenager was expected to face four counts of murder and other charges.

There was no immediate information about a motive, but the statement described the shooting as an isolated domestic incident. Calls and emails to the prosecutor’s office early on Monday were not immediately returned, and the teenager was not named.