TULSA, Okla. — A state lawmaker called Thursday for an independent review of Tulsa’s reserve deputy program as more questions arose about the training of a 73-year-old volunteer officer who says he accidentally shot a suspect to death while the man was being held down by others.

The Tulsa County Sheriff’s Office said it would conduct an internal investigation, but Rep. Mike Shelton fears a growing mistrust of police after a series of high-profile killings by law officers around the country.

“There are too many variables here for me and anyone else to believe that this investigation is going to be impartial,” said Shelton, a Democrat from Oklahoma City. “We simply cannot afford to have the public’s trust in our law enforcement eroded any further. We need to remove the appearance of personal and political biases from this case.”

Reserve Deputy Bob Bates has said he mistakenly pulled out a handgun rather than a stun gun on April 2 as Eric Harris lay on the ground after running from deputies conducting a sting operation. Video from the scene captured Bates apologizing for shooting Harris, who was being detained on suspicion that he tried to sell guns to an undercover officer.

“Oh, I shot him! I’m sorry,” said Bates, who has been charged with second-degree manslaughter.

Bates, an insurance executive, has been a volunteer reserve deputy since 2007 or 2008 and served as Sheriff Stanley Glanz’s re-election campaign manager in 2012. He has also been a generous donor, giving cars and equipment to the sheriff’s office.

Harris’ family has questioned whether Bates was sufficiently trained. Glanz has said the department cannot locate some of Bates’ certification records, and the Tulsa World reported Thursday that sheriff’s office supervisors who refused to fudge Bates’ paperwork were transferred to other jobs. The newspaper cited sources that it did not identify, and it did not say when the transfers occurred.

In a video interview posted on the newspaper’s website, Tulsa World reporters said their sources told them Bates did not perform well enough at a shooting range to be certified as a law officer but that supervisors were told to approve his performance anyway.