Hillary Clinton should not be charged over her use of a private email servers while she served as secretary of state, the FBI has said.

FBI director James Comey said at a Tuesday press conference that the investigation turned up "reasonable confidence that there was no intentional misconduct," but that there "is evidence that [she and her staff] were extremely careless with highly sensitive classified information."

"We cannot find a case where we would bring a prosecution based on these facts," said Comey. "No charges are appropriate in this case," he added.

His statement was in response to a long-running investigation by the FBI to determine if there was evidence that classified information was improperly stored or transmitted through a private email system used by Clinton.

Comey was nonetheless critical of the former secretary's practices, even no charges were recommended.

"None of these emails should have been on a non-classified servers," said Comey.

Comey confirmed during his statement that more than one email server was used by Clinton. One server was decommissioned as late as 2013.

Those servers were "generally lacking" in strong security mechanisms, compared to government systems, he said.

But Comey said though the FBI didn't find "direct evidence" that her private email domain was hacked successfully by nation states, the agency couldn't rule it out entirely.

The investigation showed that 110 emails in over 50 separate email chains were considered "classified" at the time that they were received. Eight of those chains that was considered "top secret," the highest level of standard classification.

The news will likely rally both political support, and considerable criticism -- not least from her presidential campaign counterparts.

Critics in particular accused the Democratic presidential hopeful of skirting government archiving and transparency laws.

A report issued by the State Department's Inspector General earlier this year made similar conclusions, arguing that staffers though a private email server would be outside the scope of federal records keeping laws. The inspector-general also found "longstanding, systemic weaknesses" relating to electronic records keeping, archiving, and poor cybersecurity practices.

Clinton was voluntarily questioned by federal investigators as recently as this weekend.

However, when she was approached by State Department watchdog staff investigating communications at the government agency, she declined to participate.