Thomas Matthew Crooks' Parents Called Cops to Report Him Missing Day of Shooting

Thomas Matthew Crooks trying to kill Donald Trump seems to have come out of left field -- 'cause his parents didn't know where he was that day ... and called the authorities about it.

According to FOX News ... the gunman's mom and dad were looking for their 20-year-old son Saturday not long before he showed up in Butler, PA for the Trump rally -- only to perch himself on a roof and open fire on the ex-Prez, which cost him his life.

Thomas' mom and dad reportedly called the authorities about his MIA status BEFORE he opened fire at the rally ... which left retired volunteer fire chief Corey Comperatore dead.

Per the outlet, Thomas' parents are now cooperating with the FBI, who are still trying to determine the shooter's motive for Saturday's attack.

Still, Thomas reportedly had every intention of surviving the ambush ... as CNN reports he told coworkers he'd be back to work on Sunday after requesting Saturday off -- allegedly telling them he had "something to do."

According to the report, Thomas first raised suspicions 3 hours before the shooting ... as he entered the security screening area holding a rangefinder -- which is often used by hunters to measure distance.

However, the rangefinder alone was not enough to keep Thomas from passing through security ... though security personnel reportedly kept an eye on him in the area. In other words ... they were aware of Thomas' suspicious presence PRIOR to the shooting.

Per CNN, investigators discovered 2 remote-controlled explosive devices in Crooks' car following the shooting.