The state medical examiner's office ruled the death an accident, according to a release from the Worcester District Attorney's office.
A friend found Fidrych, 54, beneath a 10-wheel dump truck on Monday at his Northborough farm.
"He appeared to have been working on the truck when his clothes became tangled in the truck's power takeoff shaft," District Attorney Joseph Early Jr. said in a statement.
A power takeoff shaft is a driveshaft that spins at high speed and can be used to power another piece of equipment.
The investigation is now closed, a spokesman for the DA said.
Fidrych went 19-9 with a 2.34 ERA for the Detroit Tigers in 1976 to win the AL Rookie of the Year award, becoming as famous for his mop of curly hair and on-the-mound antics as his skills.
The Bird, as he was known, was beloved by Tigers fans for appearing to talk to the ball and smoothing out the mound between innings.
He never matched the heights of his first season, as injuries derailed his promising career.
He attempted a comeback with the Boston Red Sox in 1982 and 1983, but never again pitched above the Triple-A level.
Fidrych remained a popular figure in his home town, known for his friendly demeanor and generosity.
Visitation is scheduled for Thursday at the First Parish Unitarian Church in Northborough. A funeral service is scheduled for 11 a.m. on Friday at the same church, followed by private burial.
Fidrych is survived by his wife, Ann, and daughter, Jessica.
Copyright 2009 The Associated Press.