The FBI will not recommend criminal charges in its investigation into Hillary Clinton's use of a private email server while she was secretary of state, the bureau's director says.

James Comey made the announcement Tuesday, three days after FBI agents interviewed Clinton — now the presumptive Democratic nominee in the race for the White House — in the final step of its investigation.

"We cannot find a case that would support bringing criminal charges," Comey said at a news conference in Washington after describing the "painstaking" investigation.

"Our judgment is that no reasonable prosecutor would bring such a case."

But Comey also said Clinton and her colleagues at the Department of State had been "extremely careless" with classified material — noting that 110 emails, in 52 different email chains, contained classified information when Clinton sent them. Eight of those chains contained top secret information, he said.

"None of these emails should have been on any kind of unclassified system," he said.

Agency recommends no charges against Hillary Clinton following email probe 2:17

Comey also said it's possible that "hostile actors" might have hacked into Clinton's servers, and that the former secretary of state checked, sent and received work-related messages from her personal email while in foreign countries.

The Justice Department has been looking into whether anyone mishandled classified information that flowed through Clinton's email server, and whether emails relevant to the investigation were deleted. Mishandling classified material is a felony under U.S. law.

Clinton provided some 30,000 emails to investigators. Many others had been deleted but were recovered, Comey said.

Not enough to recommend a charge

He said the case lacked the "clearly intentional and wilful" mishandling of information, or the exposure of "vast quantities" that would warrant a recommendation of charges.

Attorney General Loretta Lynch said last week that she would accept the recommendations of Comey and career prosecutors.

The Clinton team, meanwhile, said they were "pleased" the FBI recommended no further action by the Justice Department.

"As the Secretary has long said, it was a mistake to use her personal email and she would not do it again," Brian Fallon, Clinton's campaign spokesman, said in a statement. "We are glad that this matter is now resolved."

Comey's announcement came as President Barack Obama campaigned with Clinton for the first time on Tuesday in North Carolina.

With shirt sleeves rolled up in campaign form, Obama vigorously vouched for Clinton's trustworthiness and dedication as he declared: "I'm ready to pass the baton."

"I'm here today because I believe in Hillary Clinton," he said. "I have had a front-row seat to her judgment and her commitment."

The White House, meanwhile, declined to comment on Comey's findings, saying the investigation was not formally closed and it did not want to appear to be influencing prosecutors.

Trump alleges 'rigged' system

Although Comey's announcement removes the threat of criminal charges, it's unlikely to eliminate concerns about Clinton's trustworthiness.

"To be clear, this is not to suggest that in similar circumstances, a person who engaged in this activity would face no consequences," Comey allowed.

"To the contrary, those individuals are often subject to security or administrative sanctions. But that is not what we are deciding now."

Comey's recommendation to the Justice Department not to prosecute almost certainly won't stop Republican presidential candidate Donald Trump from continuing to make the server a campaign issue. Within moments, Trump alleged the system was "rigged" in Clinton's favour.

FBI director said Crooked Hillary compromised our national security. No charges. Wow! <a href="https://twitter.com/hashtag/RiggedSystem?src=hash">#RiggedSystem</a> —@realDonaldTrump

Clinton's personal email server, which she relied on exclusively for government and personal business, has dogged her campaign since The Associated Press revealed its existence in March 2015.

She has repeatedly said that no email she sent or received was marked classified, but the Justice Department began investigating last summer following a referral from the inspectors general for the State Department and the intelligence community.

Critical audit

The scrutiny was compounded by a critical audit in May from the State Department's inspector general, the agency's internal watchdog, which said that Clinton and her team ignored clear warnings from department officials that her email setup violated federal standards and could leave sensitive material vulnerable to hackers.

Clinton declined to talk to the inspector general, but the audit said that she had feared "the personal being accessible" if she used a government email account.

Top aides also interviewed

The Clinton campaign said agents interviewed her this past Saturday for 3.5 hours at FBI headquarters. Agents had earlier interviewed top Clinton aides including her former State Department chief of staff, Cheryl Mills, and Huma Abedin, a longtime aide who now is the vice chairwoman of Clinton's campaign.

Lynch on Friday said that she would accept whatever findings and recommendations were presented to her. Though she said she had already settled on that process, her statement came days after an impromptu meeting with Bill Clinton on her airplane in Phoenix that she acknowledged had led to questions about the neutrality of the investigation.