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, authorities said Thursday.

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

The U.S. Attorney’s Office in Los Angeles announced the charges on Thursday. The sheriff’s department was investigating the case, and took the suspects into custody on Wednesday, because the crime occurred in an unincorporated area of the county, KTLA/AP reports.

The defendants are charged with violent crime in aid of racketeering; they are allegedly connected to the Florencia 13 (F13) gang.

Sheriff Alex Villanueva said investigators brought the case to Los Angeles County prosecutors, but because District Attorney George Gascón had previously pledged to stop seeking gang enhancements in prosecutions, detectives took the case to federal authorities.

The suspects are alleged gang members Luis Alfredo De La Rosa Rios, 29, known as “Lil J”; Ernesto Cisneros, 22, known as “Gonzo” and Jesse Contreras, 34, known as “Skinny Jack” and “Flaco.” Rios’ girlfriend, 18-year-old Haylee Marie Grisham, is considered an associate of the gang.

Under federal charges, the suspects could face the death penalty, or the minimum sentence of life in prison without the possibility of parole, because Arroyos was killed during the robbery, according to the U.S. Attorney’s Office.