Roy Halladay's dad told investigators he was worried the abuse of prescription meds played a role in his son's fatal plane crash back in Nov. 2017 ... this according to new docs obtained by TMZ Sports.

NTSB officials say in 2 separate interviews with Halladay's father shortly after the fatal flight -- he told them he was worried the ex-Philadelphia Phillies ace might have been abusing Rx drugs.

Investigators claim Halladay's father told them Roy had "enrolled in an in-house detox program for an addiction to Lorazepam" a couple years prior to the accident ... and he told officials he was "concerned that Roy was abusing prescription medications, and that may have played a role in the accident."

As we previously reported, an autopsy revealed Halladay had zolpidem (the generic name for Ambien), as well as morphine in his system at the time he crashed his plane in the Gulf of Mexico.

The tests also came back positive for amphetamines.

Investigators say Halladay's dad told them Roy was suffering from anxiety and depression around the time of the accident.

According to the report, Halladay's dad also told officials he believed back pain, shoulder surgery and anxiety were the only medical ailments the former pitcher suffered from.

There's more ... officials also claim Halladay's father told them Roy often took risks while flying ... saying, "Mr. Halladay had a feeling that his son did not appreciate the dangers involved with flying."

Roy -- who was 40 at the time of the crash -- had tweeted about a conversation with his father just days before the accident, writing, "I keep telling my dad flying the Icon A5 low over the water is like flying a fight jet!"

"His response...... I am flying a fighter jet!!"

Officials also included new photos of the wreckage after it was pulled from the water -- which shows the aircraft was essentially reduced to scrap.