That’s why Vince Foster was found in Fort Marcy Park, where the DC Park police did the investigation.
Fort Marcy Park is in Virginia not DC. The Park Police are a federal not metropolitan agency. Their full title is United States park Police, they belong to the Dept of the Interior. While they are permanently based only in the national Capital Region and New York City (they are the police power on Ellis Island, Liberty Island and I think Castle Clinton (not named for Bill) at the tip of Manhattan Island the Park Police can be deployed to any national park or monument if needed. That said they have a very limited criminal investigative capacity and that is certainly why they were retained as lead investigative agency in the Vince Foster case rather than having the case be transferred to the FBI, which would be routine in any case of homicide occurring on federal ground.