CHICAGO (CBS) — The Guardian Angels have been passing out fliers at the Garfield CTA Red Line stop – fliers that show photos of the four young men believed to have robbed and beaten a 21-year-old man on the Red Line three weeks ago.

On Dec. 7, a young man asked a Red Line passenger for a donation to a basketball team and when the passenger gave him $5, he demanded more and was joined by three other young men who beat the passenger.

READ MORE: Chicago Police Union President Urges Aldermen To Repeal Mayor's Vaccine Mandate For City Workers, Judge Denies Request To Extend Gag Order

The Guardian Angels passes out flyers with surveillance photos of the four robbery suspects.

“We want to make sure we catch them before they rob someone else,” says Miguel Fuentes, national director of the Guardian Angels.

READ MORE: Chicago Weather: Most Locations To Remain Dry Overnight

Police say the robbers left the train at Garfield.

MORE NEWS: Illinois State University Student Jelani Day's Death Ruled A Drowning

To leave an anonymous tip with the Guardian Angels, call 312-217-7245.