MILAN (AP) — Italian police have arrested a fourth suspect, an immigrant without legal documents, in the murder of a teenage girl in a Rome drug dealers' den.

The slaying has underlined the degradation and lawlessness in some areas of the Italian capital and put the spotlight on illegal immigration.

Interior Minister Matteo Salvini said a Gambian was arrested in the case Friday. He pledged that he and the other three suspects, also African immigrants without documents to live in Italy, would see "hard prison and then home!"

The four men are being held on charges of murder, sexual violence and distributing drugs.

Police say 16-year-old Desiree Mariottini had been drugged, gang raped and then left in an abandoned building used by drug dealers. Residents have protested the general neglect of the neighborhood.