"In looking back at our investigation into the mishandling or removal of classified information, we cannot find a case that would support bringing criminal charges in these facts," FBI Director James Comey said in a press conference Tuesday.
Still, Comey described the behavior of Clinton and her colleagues as "extremely careless in their handling of very sensitive, highly classified information."
The FBI's recommendation regarding prosecution to US Department of Justice is the latest development in the long-running drama over Clinton's use of private email servers, which prompted many to question whether she or her aides had broken any laws. It's an issue that has dogged her in the presidential race and is a favorite topic of Republican front-runner Donald Trump.
Comey was surprisingly forthcoming with the details of the investigation. "This is going to be an unusual statement in at least a couple of ways," he said.
Classified info in an unclassified system
The FBI found more than 100 email chains containing some level of classified information, according to Comey.
He also said the culture of the State Department at the time was generally lacking in the kind of care that's found elsewhere in the government regarding classified information.
"A reasonable person in Secretary Clinton's position, or in the position of those government employees with whom she was corresponding about those matters, should have known that an unclassified system was no place for that conversation," Comey said.
Clinton's actions could have exposed sensitive details about US government operations. The possibility existed, for instance, that circumstances might have allowed hackers to gain access to her correspondence, if the people Clinton corresponded with had been hacked themselves, Comey said. The agency, however, found no evidence of that on her servers.
Comey also talked about the process of not only reading more than 30,000 emails, but piecing together millions of email fragments from deleted, unarchived emails.
Several thousand work-related emails were not turned in by the Clinton camp. Comey said that the FBI did not think those emails were intentionally deleted and that there was no misconduct in the sorting of those emails.
The Clinton campaign expressed relief over the findings.
"We are pleased that the career officials handling this case have determined that no further action by the Department is appropriate. As the Secretary has long said, it was a mistake to use her personal email and she would not do it again. We are glad that this matter is now resolved," said Clinton campaign spokesman Brian Fallon in a statement.
Lynch on Wednesday released a statement, saying she had met with "Comey, career prosecutors and agents who conducted the investigation," earlier in the day. "I received and accepted their unanimous recommendation that the thorough, year-long investigation be closed and that no charges be brought against any individuals within the scope of the investigation."
Updated July 6 at 3:15 p.m. PT: Added statement from Attorney General Lynch.