they can't get along with each other.
In the five years since the FBI and ATF were merged under the Justice Department to coordinate the fight against terrorism, the rival law enforcement agencies have fought each other for control, wasting time and money and causing duplication of effort, according to law enforcement sources and internal documents.
There've been bits and pieces about this coming out over the years, including FBI having fought like hell, from what I've read, to keep ATF from being stuck in with them. According to this, local agencies get the amusement of seeing two groups of feds threatening to arrest each other at crime scenes. Sometimes while they're each busy trying to take it away from local or other federal authorities.
"If you're working with one agency, you have to walk on eggshells if you mention the other," said Jeff Kirk, former commander of the Kokomo, Ind., police bomb squad, who has written to Congress about the issue. "Frankly, after all these years, I'm really tired of this alphabet soup fight."
Found through the Boomershoot guy.