SAN JUAN, Puerto Rico (AP) — Five people were killed in a drive-by shooting in the U.S. territory of Puerto Rico, forcing authorities to close a portion of a highway, police said Tuesday.

Three of the victims were found dead inside a pickup in the eastern coastal town of Ceiba, while a fourth was found on the pavement. A fifth victim died at a hospital while being treated for his wounds, according to a police statement.

One of the victims is a 16-year-old boy, Police Commissioner Antonio López told WKAQ radio station. He said weapons were found inside the bullet-riddled pickup, and that the victims had just left a cock-fighting establishment in the nearby town of Naguabo.

It wasn’t immediately clear what led to the shooting that occurred late Monday, although López said authorities believe it is related to drug trafficking.

At least 28 people have been reported killed so far this year on the island of 3.2 million people, compared with 30 in the same period last year.