Find another attorney. When calling law firms, find out if they have any experience with the Federal Tort Claims Act because it was a Dept. of Homeland Security-ICE vehicle that hit you. Some law firms may not want to bother, but with a few phone calls, you should be able to find one with an attorney that has experience in this area.
Believe me, Postal and Federal vehicles have been crashing into private vehicles (and vice versa) for decades. The Post Office may now be considered a private company for the Federal Tort Claims Act, but you still should be able to locate a law firm with an attorney that has some experience with these claims. This kind of accident is not "that rare".