Among the agents who are no longer clocking hours ... the head of the Pittsburgh field office ... according to a report Friday, citing a source familiar with the matter.

Trump, as you know, was holding an outdoor rally July 13 in Butler, PA when Thomas Matthew Crooks scaled the roof of a nearby building and squeezed off 8 shots at the former president ... grazing Trump's ear, killing a man in the crowd and wounding 2 others.

Videos show Crooks casing the building, walking around the fairgrounds and taking up a position on the roof ... only a few hundred yards from Trump.

Crooks also flew a drone over the event, and was on law enforcement's radar ... but somehow he evaded cops and the Secret Service. In other words, this was a massive security failure.

Secret Service Director Kimberly Cheatle resigned in the wake of the assassination attempt amid mounting calls for her to step down ... and a Congressional investigation into what went down in Butler.

Separate from Congress, the Secret Service's internal affairs department continues to probe the would-be assassination.