A State Department spokesman argued Wednesday that some of the sensitive emails recovered by the FBI from Hillary Clinton's server were only marked classified by accident in the agency's first public acknowledgement that its past statements on the designation of records in Clinton's inbox were inaccurate.

Spokesman John Kirby told reporters that the classified markings discovered on some of Clinton's emails were the result of "human error" and not of negligence during a briefing held just one day after the FBI accused Clinton and her team of "extremely careless" treatment of classified information.

The State Department and Clinton herself have consistently argued that nothing circulated on the former secretary's private server was marked classified at the time.

But FBI Director James Comey debunked those statements Tuesday when he announced that not only were some of the emails in Clinton's inbox marked classified, but the presence or absence of classified markings is an irrelevant distinction when it comes to the protection of sensitive material.

The New York Times reported Wednesday that the classified emails to which Comey was referring included call sheets an aide had prepared for Clinton ahead of telephone conversations with foreign leaders. Those documents bore classified markings, a detail that contradicts both Clinton's characterization of her email use and past assertions made by the State Department denying the presence of any records marked classified on the former secretary of state's network.

"The classification on them are not fixed in time," Kirby said of the call sheets.

"Call sheets are often marked ... at the confidential level prior to a decision by the secretary that he or she will make that call," he added, noting that the classification markings are typically removed after the secretary decides whether to contact the foreign leader in question.

But Kirby could not answer questions as to why, if the markings are typically removed from call sheets after the information within them is no longer considered classified, those records ended up in Clinton's inbox with the markings intact. That detail would suggest the call sheets were emailed to Clinton when the information in them was still considered classified.

Kirby also refused to answer questions about how many more emails on Clinton's server were marked classified.

Brian Fallon, a spokesman for Clinton's campaign, quickly seized on Kirby's argument.

Republicans have been rankled by the FBI's decision not to recommend criminal charges for Clinton and her aides despite evidence that material marked classified ended up on the former secretary's unsecured server.