Sen. Chuck Grassley (R-Iowa) on Wednesday pressed Attorney General Eric Holder for more information about the FBI’s use of drones and questioned whether he has been holding back information from Congress.

Grassley, the ranking member on the Senate Judiciary Committee, asked Holder to provide more information about the number and types of drones used by the FBI, the rules that govern their use, and whether they’re capable of being armed.

The letter to Holder comes after FBI Director Robert Mueller testified earlier on Wednesday — in response to questions from Grassley and Sen. Dianne Feinstein (D-Calif.) — that the FBI uses drones to conduct surveillance operation within the U.S.