Bates is the volunteer deputy who fatally shot suspect Eric Harris instead of stunning him with a Taser on April 2.
A Harris family lawyer provided documents to CNN from the 2009 inquiry, which also found that supervisors intimidated employees to disregard policies to the benefit of Bates.
The Tulsa County Sheriff's Office has not responded to repeated calls for comment from CNN.
An attorney for Bates, Clark O. Brewster, told CNN's Anderson Cooper on Friday that he disputed the findings in the report and maintained that his client had the proper training.
Bates, 73, is charged with second-degree manslaughter in the shooting of Harris. He pleaded not guilty.
Bates was working as a reserve deputy for the Tulsa County Sheriff's Office on April 2 when he was involved the arrest of Harris in a weapons sting operation.
Bates has said he meant to stun Harris with a Taser after the suspect fled from officers but mistakenly shot Harris with a gun instead.
Bates has said the shooting was accidental. He has apologized to the Harris family, as has Sheriff Stanley Glanz.
The attorney for the Harris family said that Bates wasn't qualified to be on the force, but received preferential treatment because he'd made donations to the agency and was a friend of the sheriff.
The Tulsa World newspaper reported
some supervisors in the Tulsa County Sheriff's Office were told to forge Bates' records and were reassigned when they refused.
The sheriff's office denied the allegations in the newspaper's report. It also declined a CNN interview to respond to the claims.
Over the weekend, one of Bates' attorneys released most of his training documents and said they prove Bates had proper law enforcement training. However, the records were incomplete.
Bates' attorney, Brewster, characterized the 2009 memo as something borne out of jealousy by other officers.
The allegations made in the report are the result of misunderstandings and misstatements, he said.
The memos obtained by CNN show that a "special investigation" was launched in July 2009 at the request of then-Undersheriff Brian Edwards.
Edwards asked investigators to look at two questions specifically: Was Bates being treated differently than any other reserve deputies, and were any employees pressured by supervisors to aid Bates?
The investigation found that some employees felt pressured to sign off on certificates for training that Bates had not completed.
On Friday, the Tulsa County district attorney's office said it had received new information -- likely the 2009 memo -- that "are worthy of further investigation beyond the scope of the manslaughter case."
The DA's office is reaching out to independent law enforcement agencies to investigate further.
Allegations of special treatment
Contrary to claims by the sheriff's office that Bates had the required training to be in the field, the 2009 memo indicates otherwise.
The document names then-Chief Deputy Tim Albin (currently undersheriff) and then-Capt. Tom Huckeby (now a major) as two supervisors who allegedly pressured lower-ranking officers to make exceptions or falsify records for Bates.
One deputy reported that he was tasked with providing field training for Bates. The written policy is that a reserve deputy is required to have 480 hours of training, but that Huckeby and Albin pressured the trainer to write that Bates was qualified after only 320 hours.
The trainer, fearing reassignment if he did not comply, signed a memo stating that Bates had completed 328 hours of training, and did not elaborate on details of the training.
Later, the memo was amended by his superiors to read, in part, that Bates was "capable of performing the functions of a patrol deputy." The training deputy said he initialed the changes, even though he didn't think Bates was properly trained.
According to the investigation memo, the trainer said that if he was honest, he would have said Bates needed remedial training, and that the reserve deputy was "not really good at traffic stops or operations."
The internal investigation only turned up 72 hours of documented training. The trainer maintained that he oversaw 328 hours of training, but that the records were sent not to the records office, but instead to Huckeby.
Brewster denied the claims made by the trainer, and alleged that trainer lost his job in part because of his conduct during the internal inquiry.
Another deputy said that she signed a driving certificate for Bates, even though she believed he had not completed the training. Albin asked the deputy to make the certificate, and she did so without questioning him, according to the investigation.
The memo lists other instances where Bates performed operations that he wasn't supposed to, given his stature in the department, but that complaints by other officers were dismissed by supervisors.
Those who complained about Bates, according to the memo, were told to leave him alone or to make an exception, citing that he did a lot of good for the county and was close with the top leaders in the agency.