The shocking claim comes in body cam footage of the chaotic scene right after Trump took a bullet to the ear, and a Secret Service sniper took down the shooter. Two officers are discussing the shooting ... and one says he gave the feds ample warning.

The unknown officer says, "I f***ing told the Secret Service, post a f***ing guy over here. I told them that s*** at the meeting on Tuesday." The cop's referring to the area around the building Crooks climbed up on.

As we've reported, this body cam -- released Thursday -- also includes a clip of one of the officers hoisting himself onto the building where Crooks lay holding his gun, quickly dropping to the ground and racing to his car to grab a high-powered weapon.

Crooks was taken down almost immediately by a Secret Service sniper after he opened fire ... and, the officer came back and found other law enforcement personnel circling Crooks' dead body.

If the officer did warn the Secret Service, it's another major lapse in security.

Remember, the Secret Service has already been raked over the coals for not responding fast enough to civilians pointing out Crooks on the roof, and for allowing the shooter to get so close to his target. The agency also said the building was not within its perimeter, and should've been covered by local cops.