From Guns.com:

“The ATF is not an ‘agency’ within the meaning of the FOIA, 5 U.S.C. § 552(f)(1), and is, therefore, not a proper party defendant,” the agency wrote in its answer to a complaint filed in federal court in June.

Attorney and Second Amendment scholar, David Kopel, said he didn’t know the reasoning behind the ATF answer. “I have no idea what ATF’s argument on that point is,” he said. “There is no separate FOIA law for ATF, and they are plainly an agency within the meaning of the statute.”