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Clinton is doing her best to move beyond the controversy over her use of a private email server as secretary of state, saying only last night that she "made a mistake" and declining to address why those emails were deleted. She didn't answer Trump's questions. However, there is an answer.
The bottom line: Clinton believed the deleted emails were not work-related.
In late 2014, the State Department asked Clinton and other former secretaries of state to hand over any work-related emails they may have.
By then, Clinton had already "deleted some [emails] over time as an ordinary user would," FBI Director James Comey told lawmakers at a July congressional hearing.
And she tasked her legal team to determine which of the roughly 60,000 emails still on her server were work-related.
"Clinton told the FBI that she directed her legal team to provide any work-related or arguably work-related emails to State; however she did not participate in the development of the specific process to be used or in discussions of the locations of where her emails might exist," the FBI concluded in its investigative summary of the case.
Comey testified that the FBI "didn't find any evidence of evil intent and intent to obstruct justice."
To determine which emails were work-related, a member of Clinton’s legal team did four things: she automatically deemed any email sent from or to a .gov and .mil address as related to work; she searched the tens of thousands of emails for names of senior State Department officials, lawmakers, foreign leaders and other government officials; she conducted a keyword search for work-related terms; and she looked at the sender, recipient and "subject" of every email for other potentially work-related emails, but she did not read the contents of those emails.
In December 2014, Clinton’s legal team provided about 30,000 emails -- totaling 55,000 pages -- to the State Department.
"[Clinton] then was asked by her lawyers at the end, 'Do you want us to keep the personal emails?' And she said, 'I have no use for them anymore.' It's then that they issued the direction that the technical people delete them," Comey told lawmakers.
In total, more than 30,000 emails were deleted "because they were personal and private about matters that I believed were within the scope of my personal privacy," Clinton told reporters in March of 2015, as the controversy around her private emails was growing.
"They had nothing to do with work," Clinton added. "I didn't see any reason to keep them ... no one wants their personal emails made public, and I think most people understand that and respect that privacy."
Clinton said her team "went through a thorough process" to identify work-related emails, and she said he had "absolute confidence that everything that could be in any way connected to work is now in the possession of the State Department."
However, after a year-long investigation, the FBI recovered more than 17,000 emails that had been deleted or otherwise not turned over to the State Department, and many of them were work-related, the FBI has said.
Republican critics have accused Clinton of using a private server and ultimately deleting emails to skirt federal records laws aimed at promoting transparency, and earlier this month the chairman of the House Oversight and Government Reform Committee, Jason Chaffetz, R-Utah, said, "The sequence of events leading up to the destruction of Secretary Clinton’s emails ... raises questions about whether Secretary Clinton, acting through her attorneys, instructed [aides and others] to destroy records relevant to" a Congressional investigation of the deadly Benghazi attack.
At the July hearing before Chaffetz's committee, Rep. Jim Jordan, R-Ohio, asked Comey, "Did Secretary Clinton know her legal team deleted those emails that they kept from [Congress]?"
"I don't believe so," Comey responded.
During Monday night's debate, Trump promised to release his tax returns "when she releases her 33,000 emails that have been deleted," adding that he would "go against" the advice of his lawyers "if she releases her emails."
Moderator Lester Holt then asked Trump if that meant releasing his tax returns was negotiable.
Trump responded: "It's not negotiable. No, let her release her emails. Why did she delete 33,000 emails?"