Mariottini lived in the town, situated south of Rome, with her mother and grandparents.
Her body was discovered in a building used as a squat in the Roman district of San Lorenzo on October 19, two days after she called her grandmother to say she’d missed the last bus home and would be staying with a friend overnight.
San Lorenzo is known for its vibrant nightlife and for being a hotspot for drug dealing.
Investigators believe Mariottini fell unconscious for several hours after being given drugs by several people, and died of an overdose.
She was gang raped before she died.
Rome Mayor Virginia Raggi last week announced a day of mourning for the Italian capital to coincide with the teenager’s funeral, while also imposing a ban on alcohol in San Lorenzo from 9:00 pm every night.
“Rome wants to shows its solidarity with Desirée’s family,” she said.
“We all feel their profound grief.”
Two Senegalese men aged 26 and 43 were held overnight last Wednesday, and a 40-year-old Nigerian was detained last Thursday.
A fourth suspect, a Gambian national, was arrested on Friday in Foggia.
The man was caught in possession of around 10 kilograms of marijuana, police said.
It is expected the four suspects will be charged with group sexual violence, voluntary manslaughter and drug dealing.
Police believe there may have been more people present at the scene, all of whom failed to call for help or intervene.
