Oklahoma's top investigative agency said Monday it has opened an inquiry into allegations of misconduct within the Tulsa County Sheriff's Department following the fatal shooting of a restrained man by a volunteer deputy and the release of a 2009 memo that raised concerns about the volunteer's training.

The Oklahoma State Bureau of Investigation said in a news release it is investigating the April 2 incident, when reserve deputy Robert Bates, who has said he confused his stun gun with his handgun, shot Eric Harris. Bates has since resigned from the reserve deputy program and pleaded not guilty to second-degree manslaughter. Bates is white and Harris was black, but the victim's brother has said he does not believe race played a role.

After the shooting, a memo surfaced indicating that officials within the sheriff's office pressured staffers to ignore Bates' lack of training.

The OSBI said it launched the investigation at the request of Okmulgee County District Attorney O.R. Barris, who was assigned to the case by the Oklahoma Attorney General's Office after Tulsa County District Attorney Steve Kunzweiler asked that someone else be appointed to investigate the office to avoid any perception of a conflict.

The OSBI said it will present Barris with a written report when its investigation is complete and the district attorney will decide if any state laws have been violated. It did not say how long its investigation may take or whether Sheriff Stanley Glanz will be part of its focus.

Bates is a longtime friend of Glanz, donated tens of thousands of dollars in cash and equipment to the sheriff's office and was Glanz's campaign manager during the 2012 election. Glanz, a member of the OSBI's board of commissioners, has said his office will fully cooperate with the OSBI.

Harris family attorney Dan Smolen called the OSBI investigation "a positive development," but added, "there is also an element of concern." He said Glanz's relationship with the agency raises questions about whether the investigation will be independent and objective.

Separately, a local civil rights group is collecting signatures for a grand jury to investigate whether Glanz neglected his duties and whether reservists who gave gifts to the sheriff were given special treatment.

OSBI spokeswoman Jessica Brown declined to comment further on the agency's announcement. Barris also declined comment, and Tulsa County Sheriff's Office spokesman Terry Simonson did not immediately return a telephone call seeking comment.

Two top administrators in the sheriff's office have resigned since the shooting, and the office said last week that the former spokesman who was placed on administrative leave is no longer with the office.

Glanz has said the reserve deputy program has been temporarily suspended pending an internal review of the certification and training records of its 126 reservists.