The State Department has completed its internal investigation into former Secretary of State Hillary Clinton's use of private email and found violations by 38 people, some of whom may face disciplinary action.

The investigation, launched more than three years ago, determined that those 38 people were 'culpable' in 91 cases of sending classified information that ended up in Clinton's personal email, according to a letter sent to Republican Sen. Chuck Grassley this week and released on Friday.

The 38 are current and former State Department officials but were not identified.

The investigation covered 33,000 emails that Clinton turned over for review after her use of the private email account became public.

The department said it found a total of 588 violations involving information then or now deemed to be classified but could not assign fault in 497 cases.

The State Department has completed its internal investigation into former Secretary of State Hillary Clinton's use of private email and found violations by 38 people

For current and former officials, culpability means the violations will be noted in their files and will be considered when they apply for or go to renew security clearances.

For current officials, there could also be some kind of disciplinary action. But it was not immediately clear what that would be.

Although the report identified violations, it said investigators had found 'no persuasive evidence of systemic, deliberate mishandling of classified information.'

However, it also made clear the Clinton's use of the private email had increased the vulnerability of classified information.

The report concluded 'that the use of a private email system to conduct official business added an increased degree of risk of compromise as a private system lacks the network monitoring and intrusion detection capabilities of State Department networks.'

At least 340,000 emails between Clinton and top aid Huma Abedin were found when FBI searched her estranged husband Anthony Weiner's laptop for messages he'd exchanged with a 15-year-old girl

The discovery of the emails led Comey to reopen the investigation into Clinton two days before the presidential election

The department began the review in 2016 after declaring 22 emails from Clinton's private server to be 'top secret.' Clinton was then running for president against Donald Trump, and Trump made the server a major focus of his campaign.

Then-FBI Director James Comey held a news conference that year in which he criticized Clinton as 'extremely careless' in her use of the private email server as secretary of state but said the FBI would not recommend charges.

The Justice Department's inspector general said FBI specialists did not find evidence that the server had been hacked, with one forensics agent saying he felt 'fairly confident that there wasn't an intrusion.'

Grassley started investigating Clinton's email server in 2017, when he was chairman of the Senate Judiciary Committee. The Iowa Republican has been critical of Clinton's handling of classified information and urged administrative sanctions.

The report into Clinton's emails stated that although there were violations, investigators had found 'no persuasive evidence of systemic, deliberate mishandling of classified information' However, it also made clear the Clinton's use of the private email had increased the vulnerability of classified information

During her presidential campaign, FBI investigating former Rep. Anthony Weiner for having a relationship with a 15-year-old girl searched his laptop and discovered emails between his estranged wife, Huma Abedin and Clinton.

At least 340,000 emails between the former Secretary of State and her top aide were found, with many using a hillaryclinton.com address.

The investigation into Weiner's devices began when DailyMail.com revealed that the disgraced congressman had an online sexual relationship with the troubled teenager for months and sent her lewd messages and pictures.

At the time, the discovery went ignored at the FBI because senior officials were 'overwhelmed' by the investigation into Russian meddling in the 2016 election, according to Deep State: Trump, the FBI, and the Rule of Law, a book by James B Stewart.

He also said that one agent went outside the chain of command and shared the allegations with New York prosecutors, which led to Comey reopening the investigation days before the 2016 election.