Grand Jury is not required and is time consuming. The Feds can charge without a grand jury, make the case to the Bonding Judge of the serious nature, backed up by premliminary evidence, in order to continue pretrial confinement as a continuing danger. Things have started coming out, already of writings by the suspect, and proven connections overseas. Just a felon with a weapon, that has had a serial number scratched off, is usually not enough to justify denial of bond, in most cities and states. They need to move quicker and put out more of what they know. In the end all prosecutor evidence is turned over to defense, anyway.
Oh, and too my knowledge, he was engaged by Secret Service before getting a shot off, himself.