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 .