Investigations into Hillary Clinton's use of a private email server that held classified information -- as well as her inner circle's dealings -- are being hampered by revelations that their computer devices have been wiped clean or destroyed, despite the Democratic presidential candidate claiming earlier this week she doesn't know "how it works digitally at all."

Clinton's lawyer David Kendall recently told a Senate committee that emails and all other data stored on her computer server were erased before the device was turned over to federal authorities.

In a letter sent last week to Sen. Ron Johnson, R-Wis., the chairman of the Senate Homeland Security Committee, Kendall said the server was transferred to the FBI on Aug. 12 by Platte River Networks, a Denver firm hired by Clinton to oversee the device.

Yet at a press conference Tuesday, Clinton declined to say whether the server was erased.

Asked repeatedly by Fox News whether it was wiped, she joked: "What, like with a cloth or something? I don't know how it works digitally at all."

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    The FBI is apparently trying to recover the contents. An intelligence source familiar with the review told Fox News that investigators are confident they may be able to recover some of the deleted files.

    Meanwhile, in a separate case, the State Department said in a court filing that BlackBerry devices used by two former Clinton aides during her time as secretary of state have most likely been destroyed or sold off by the agency.

    The devices, used by Cheryl Mills and Huma Abedin, have not been located by the agency, State Department Executive Secretary Joseph Macmanus wrote in a filing submitted in D.C. federal court, adding that the agency's "standard procedure upon return of such devices is to perform a factory reset (which removes any user settings or configurations) and then to re-issue the device to another employee, to destroy it, or to excess it."

    "Because the devices issued to Ms. Mills and Ms. Abedin would have been outdated models, in accordance with standard operating procedures those devices would have been destroyed or excessed," Macmanus wrote.

    He also said that Clinton did not have a BlackBerry from the State Department or any other device. That means she did not have a department-issued device certified as secure, raising new security questions as the devices she carried would not have been government-encrypted.

    Wednesday's court filing came amid a lawsuit by the group Judicial Watch, which is seeking records over the employment status of Huma Abedin, the former deputy chief of staff to Clinton. Abedin has been under scrutiny for previously working for a private firm at the same time she was working for the then-secretary of state.

    "The questions just keep popping up. Every time the State Department tries to justify its stonewalling, one more bit of information arises," Judicial Watch President Tom Fitton said in a statement. "If the State Department was not providing secure email devices to Mrs. Clinton, who was? Best Buy? Target? Mrs. Clinton clearly did whatever she wanted, without regard to national security or federal records keeping laws."

    State Department spokesman John Kirby later said it is "standard practice" that when employees leave, their personal devices undergo a "factory reset," as they're often given to other employees.

    "And it's our understanding that that's what happened in this case. It's also likely that because this was a while, ago and that those devices were probably -- may have been destroyed," he said.

    A federal court has scheduled a hearing for Thursday afternoon on the Abedin case.

    Meanwhile, the Clinton campaign is mounting a vigorous defense of the candidate's email practices.

    On Wednesday, the campaign tried to use a Fox News report on the origin of the FBI probe into the candidate's server in a bid to argue it proves she did nothing wrong -- though a top government watchdog pushed back on the campaign's claims.

    The exclusive report identified emails that helped kick-start the current investigation. The emails were from top Clinton advisers, including Abedin, and had earlier been released to the Benghazi select committee.

    On the conference call Wednesday reacting to the report, top Clinton campaign aides said those emails were not marked classified at the time they were sent.

    However, despite the Clinton campaign's claims, a spokeswoman for the intelligence community inspector general reiterated to Fox News that the information in the emails was in fact considered classified at the time it was sent.

    Fox News' Catherine Herridge and The Associated Press contributed to this report.