Mulder and Scully are asked by the Philadelphia Police Department to assist in their investigation of the murder of Joseph Cutler, however the investigation of his murder would not fall under the jurisdiction of the local police. Cutler was an employee of the United States Department of Housing and Urban Development (HUD), which is a department of the executive branch of the federal government, making him a government employee. The murder of an employee of the federal government would automatically fall under the jurisdiction of the FBI.
When the forensic tech says there are no prints to be lifted, Fox Mulder states that that is not possible. It is in fact possible, with the adermatoglyphia genetic mutation, some families are born without finger or foot prints.
Cutler tells the homeless group that the HUD office is at 1804 South Walnut Street. But only a few minutes later, when he calls the police from his office, he tells them he's at 1809.
Cutler tells the homeless that the HUD office is located at 1809 South Walnut Street. Walnut Street in Philadelphia runs East to West, there is no "south" part to Walnut Street.