Yes.
Here's the part everyone forgets about shootings:
1) You need a familiarity with a gun. How to aim, how to handle recoil, how to reload. The Orlando shooter, for example, clearly had practice, former G4S. Who taught him?
2) Mentally ill people don't think straight. Anyone with PTSD wouldn't really be able to handle a gun. To coordinate, plan, and execute an attack requires a clear mind with enough cognitive ability to factor in contingencies, building layout (how'd they get blueprints or access in the first place when it comes to secured or unusual locations EG YouTube building? Does YouTube's building not have biometric security or even simple locks?).
3) Being able to focus in a scene of chaos is not something a lot of introverts can do. Most introverts shun any sort of noise. Saying 'autistic' and 'handgun' is an oxymoron because autistics shun loud noises. Anyone with a mental sensibility is not going to like the loud sound or smell of gunpowder. That takes practice. How do they suddenly overcome what is literally titled a disability? Hypnosis?
4) Former veterans or prior security work makes more sense because it removes the need for training. Fort Hood comes to mind - do they need counselling if the Microwave Auditory Effect ('voice of god' tech) works?
5) Weapons need money. How do broke-ass disabled people with no job always have backpacks worth of ammo? That shit costs thousands. Who has ammo to spare? The army. Who has weapons to spare (EG 10+ AKA Las Vegas shooting)? The army.
6) The mentally ill person is never seen down any gun ranges practicing. They also never have their own home, and no-one ever comments about seeing them practice. See point 1. How did the Las Vegas shooter know how to aim a weapon over long distance from a raised building? Urban warfare is only taught in the army, civilians don't know jackshit on it.
7) If a mentally ill person is flagged up to an agency, and continuous surveillance is an easy thing to achieve, why do agencies never somehow notice the shit ton of weapons and ammo being prepared? Or indeed, any of the always public, NSA metadata searchable manifestos?
8) Why do the majority of the shooters have indirect ties to the military? Orlando: G4S. Ronald Reagan attempt: US navy. Fort hood: US navy. John Lennon: USAF. Aurora: DARPA. JFK: USAF/CIA. MLKJ: FBI.