The UCLA psychiatric hospital where Robert Carradine attempted suicide failed to follow key safety rules designed to protect patients like him ... according to an investigative report from a federal government agency.

TMZ obtained the newly filed report from the Department of Health and Human Services, which found UCLA's Resnick Neuropsychiatric Hospital was not in compliance with federal requirements for patient rights and nursing services.

Investigators say the hospital failed to follow its policy of checking in on Robert every 15 minutes as required ... and found that staff also failed to keep contraband out of his room.

That included the belt Robert ultimately used in his suicide attempt ... along with a bedside table.

UCLA's Director of Quality Management told investigators the admitting nurse failed to confiscate Robert's belt when he arrived -- his cellphone and wallet weren't taken either.

She also said the bedside table was likely left behind by a previous patient ... and she didn't know who was responsible for removing it.

As TMZ previously reported ... Robert's family sued the university regents earlier this month ... alleging UCLA failed to protect the "Revenge of the Nerds" and "Lizzie McGuire" actor after he checked into the psychiatric unit.

Robert was found with the belt around his neck and revived, but suffered severe oxygen deprivation to his brain. His family took him off life support two weeks later.

Robert died February 23 at 71 ... and the medical examiner ruled his death a suicide by hanging.

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