The US Marshals are not the only federal law enforcement agency doing something like this. According to documents I obtained through a FOIA in 2012, ICE has purchased an airbourne mounting kit and paid for airbourne training for their Stingray II cell phone tracking gear. See: https://www.documentcloud.org/documents/479397-#document/p44

Anyone interested in learning more about IMSI catchers and their use by US law enforcement agencies might be interested in this law review article I wrote. http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2437678