Thomas Matthew Crooks Was Mostly Unknown to Gun Club, But Visited with Father

Thomas Matthew Crooks was a member at a local gun club in Pennsylvania -- and even went a day before firing upon Donald Trump -- but he didn't leave a lasting impression on very many people who regularly go there ... TMZ has learned.

We've called around and connected with a handful of current members at the Clairton Sportsmen's Club -- not far from Crooks' home -- and the answer we got from just about all of them was the same ... nobody had ever heard of this guy before Saturday's shooting .

Bob Wolanin -- a longtime Clairton member -- tells us the name and face didn't stand out to him at all ... and ditto for another member and even a current worker at the club -- who didn't want their names out there -- but who relayed a similar sentiment ... namely, Thomas was a ghost as far as they're concerned.

A separate ex-member of Clairton did tell TMZ that some who frequent Clairton -- and who actually recall seeing Thomas there -- say he had been seen dropping in with his dad ... but it's unclear how often they went, or if Thomas ever came on his own.

Reports say Thomas did, in fact, hit up Clairton on Friday -- almost 24 hours before he tried putting a bullet through Trump's head -- and practiced shooting on his father's AR-15.

Even if Thomas did go to the range a lot ... his presence wasn't felt -- not only did ordinary members not remember much of him ... but the club's top brass say they're not familiar with him either as a standout range-goer.

Clairton's president Bill Sellitto tells TMZ, "We didn't know him" -- and his wife Debbie , who also does work for Clairton, echoes that ... explaining to us that out of the many members she's spoken to since -- she hasn't come across one who knew or interacted with TMC.

Debbie says there are over 2,000 members at their club -- and so far, not one has come forward to them to suggest there were warning signs. She says he was likely a loner at their facility ... the way he apparently was in his regular life, based on eyewitness descriptions.

Obviously, Crooks was on a mission to kill that day -- in addition to his rifle ... he reportedly brought explosives and other equipment to carry out his task, even though he told his job that he'd be back Sunday , after requesting Saturday off.

More questions than answers at this point ... including why he may have done this.