Mitt Romney's sister-in-law left a handwritten suicide note tucked inside a Book of Mormon and had Xanax in her system when she died ... according to the L.A. County Medical Examiner.

The ME's report says detectives discovered a Book of Mormon on the front passenger seat of Carrie's car. In the final pages of the book, authorities say Carrie left a handwritten suicide note. Medications were also found inside the vehicle.

The report states Carrie had 6.3 ng/mL of Xanax in her system at the time of her death. A witness told first responders Carrie was seen pacing on the top level of the parking structure, watching security cameras, and looking over the edge of the parapet. The report also states surveillance footage captured her final moments.

The medical examiner's findings note the injury occurred when she fell backward from a seated position on the rooftop parapet.

As we reported ... Carrie died from blunt traumatic injuries after falling from the rooftop of a parking structure in Valencia, California, back in October.

Carrie's husband, Scott Romney -- Mitt's brother -- had reported her missing to the Sheriff's Department. He told authorities she previously drove her car off a cliff two years earlier and had struggled with anxiety, per the report.

At the time of her death, Carrie was also in the middle of a divorce. Scott filed in June, citing "irreconcilable differences." The couple married in 2016, and Carrie was Scott's third wife.