Basically, it's a modern day "posse" made up of individuals from Federal law enforcement (FBI, Secret Service, ICE, Border Patrol, etc) and from local or specialized law enforcement (railroad police, etc).

The officers are basically deputized as JTTF members. It's a common model that's been around for a long time. For example, the US Marshalls have a fugitive task force for picking up suspects with arrest warrants and the DEA has something similar for drug raids.