LOS ANGELES (AP) — Federal prosecutors have charged four people in connection with the robbery and killing of an off-duty Los Angeles police officer and allege the suspects are tied to a street gang.

Officer Fernando Arroyos, 27, was fatally shot at about 9:15 p.m. Monday in an unincorporated area of south Los Angeles County.

Authorities have said Arroyos was shopping for a house with his girlfriend when a pickup truck approached and several suspects got out. There was an argument and gunfire was exchanged between the assailants and the officer. Arroyos’ girlfriend was not hurt.

The defendants are charged with violent crime in aid of racketeering and are expected to make their first appearances in federal court on Friday.