Hillary Clinton Defends Not Knowing State Department Classification Markings

— -- After the FBI's release of government documents last week that were part of the Hillary Clinton email probe, she told ABC's David Muir Monday that she takes "classification very seriously" and sees the release of those documents as the end of "a very difficult period."

"I have learned that trying to explain what happened made it sound to some people like I was trying to excuse it. There are no excuses," she told Muir in Cleveland.

She added, "I take responsibility. I made a mistake. I've apologized, and obviously I wish I could do differently what happened. I certainly would never do that again."

The summary of the FBI's findings from July reads, "Clinton did not recall receiving any emails she thought should not have been on an unclassified system."

Clinton told Muir that when she was the secretary of state, she relied on "people in the State Department, professionals with lots of experience and expertise who were handling this material and sending it to me," to make "their best judgments."

FBI Director James Comey has said that three email chains with Clinton included at least one paragraph marked "(C)," indicating that the paragraph contained confidential material.

He has called Clinton's handling of classified info "extremely careless" but said there's no evidence to indicate she knew she was sending or receiving classified info.

Clinton told the FBI that she did not know what the "(C)" meant at the beginning of the paragraphs and speculated it was a reference to paragraphs marked in alphabetical order — something she reiterated in ABC's exclusive interview.

"There were no headers on the thousands of emails that I sent or received. There just weren't, and the FBI has not in any way contradicted that. There were a couple of emails with a tiny 'C' in a parenthesis, which did not have a header saying that means 'confidential' in this circumstance and which the director of the FBI has said and the State Department has said those couple of emails were improperly marked, even with that. So, yes, I take classification seriously, and I think the record shows that I have," she said.

Sitting next to her running mate, Tim Kaine, Clinton said she's glad the FBI released all its notes, because she didn't want "selective cherry-picking and leaks coming from Republicans on the Hill." She and her campaign see the notes as proof of why the FBI concluded that there was no basis for further action.

Kaine contrasted Clinton's backing for the notes' release with Donald Trump's refusal to release his tax returns.

"[Trump] said, 'If I run, I promise I'll release tax returns,'" Kaine said. "This is out there because Hillary said, 'Put it out there. Let the American public see it.'"

ABC News' Mike Levine contributed to this story.