"No reasonable prosecutor would bring such a case," Comey said.
The comments triggered a fierce debate between former prosecutors, government officials and legal experts on the exact definition of "reasonable" as it pertains to Clinton's case.
"A reasonable prosecutor could bring a case because the facts clearly establish many violations of law" said Joseph E. diGenova, a former U.S. attorney for the District of Columbia appointed by Ronald Reagan. "Comey's statement that there was no intent is completely negated by the fact that Clinton used multiple private servers and devices which were unencrypted and thus she knew that classified information would be compromised."
He added: "This is simple stuff. It was clear to me that he was predisposed not to bring a case. This was a political decision, not a legal one."
Michael Mukasey who served as attorney general under the George W. Bush administration, said Comey's explanation was lacking.
"Mr. Comey didn't explain why, with evidence clearly fulfilling the requirements of the two statutes involved, no reasonable prosecutor would bring the case," Mukasey wrote in a Wall Street Journal op-ed.
In his unusual news conference, Comey revealed that 110 emails in 52 email chains had been determined to contain classified information at the time they were sent or received. Comey castigated Clinton and her colleagues for being "extremely careless" in their handling of the classified information. But he said the FBI did not find "clear evidence" that Clinton "intended" to violate laws governing the handling of classified information.
He was referring mostly to 18 USC 793, which makes it a felony for someone "entrusted" with information relating to the national defense "through gross negligence" to allow information to be "removed from its proper place of custody."
Comey noted that prosecutors weigh a number of factors before bringing charges including the strength of the evidence and how similar situations have been handled in the past.
He said that cases that had been prosecuted before involved a combination of "clearly intentional and willful mishandling of classified information," or "vast quantities of materials exposed in such a way as to support an inference of intentional misconduct."
"We do not see those things here," he said.
But Mukasey argued that Comey's decision left some "puzzled and dismayed" in part because Comey stressed intent over gross negligence. "Gross negligence," he said, rather than purposeful conduct, is enough.
DiGenova said that Comey's decision was "political" meant to help Clinton. "In doing so he permanently enshrined a double standard in U.S. law enforcement," he said.
But Abbe Lowell, an attorney who has been involved in classified information cases as a defense attorney, disagreed.
"The FBI Director could not have been more correct," Lowell said in an interview.
"The common denominator of every criminal case that has ever been brought under the classified information protection laws is that the individual under investigation intentionally sent material to someone not authorized to receive it -- such as a journalist, foreign country or a government contractor," Lowell said.
"Here it was clear from the beginning that Secretary Clinton shared whatever information she shared with people in the government -- that makes it different than any case that everybody is throwing up for comparison," he added.
Steven Levin, a former federal prosecutor and a registered Republican, says that he understands why some might take offense to Comey's phrasing as to who might be a "reasonable prosecutor" but he believes that Comey was trying to draw a distinction between those who exercise strong discretion and those who abuse it.
"Comey makes clear that even though the statute doesn't require willful intent, he does, and he thinks reasonable prosecutors should look for that. I think it's refreshing for the FBI director to exercise discretion when all too often prosecutors bring charges simply because they can, not because they should," he said.
At the end of his statement Comey -- who also served during the George W. Bush administration -- acknowledged that his decision would trigger "intense public debate" but he stressed that the investigation was done "competently, honestly and independently."
"Only the facts matter," he said, "and the FBI found them here in an entirely apolitical and professional way."
Comey is scheduled to testify Thursday
on Capitol Hill regarding the investigation.
Clinton campaign spokesman accused Republicans of playing politics based on Comey's decision.
"For weeks Republicans have said they trusted FBI Director Comey to lead an independent review into Secretary Clinton's emails, but now they are second-guessing his judgment because his findings do not align with their conspiracy theories," Fallon said in a statement.