The FBI formally confirmed Monday that it is investigating Hillary Clinton's exclusive use of a private email system while working as secretary of state. In a letter dated Feb. 2 and disclosed Monday in federal court, the FBI's chief attorney, James Baker, wrote to the State Department and officially confirmed the probe after it refused to comment on the matter last year.

"Since that time, in public statements and testimony, the Bureau has acknowledged generally that it is working on matters related to former Secretary Clinton's use of a private e-mail server," wrote Baker, reported the Hill.

"The FBI has not, however, publicly acknowledged the specific focus, scope, or potential targets of any such proceedings," he said. "Thus... we remain unable [to] provide [details about the case] without adversely affecting on-going law enforcement efforts."

The Justice Department submitted the letter Monday as part of an ongoing Freedom of Information Act lawsuit filed against the State Department by government watchdog Judicial Watch, according to Fox News.

The FBI is looking into whether Clinton knowingly transmitted classified information over her unsecured server, which experts say was most likely hacked by foreign governments. The Obama administration confirmed last month that 22 emails on Clinton's server contained "top secret" information that may have jeopardized lives.

Clinton insists that she never sent or received information that was marked classified, and said during last week's Democratic presidential debate that she is "100 percent confident [that the investigation will not turn criminal]," according to the Washington Examiner.

"This is a security review requested and carried out that will be resolved," she said.

Republican National Committee Chairman Reince Priebus said in a statement that the FBI's confirmation proves that the investigation is not a simple security review.

"The FBI's announcement confirms the investigation into Hillary Clinton's secret email server is far from the routine 'security review' she has claimed," he said. "Clinton's conduct was a severe error in judgment that grossly endangered our national security and put highly classified information at risk."

Republican Rep. Darrell Issa and former U.S. House Majority Leader Tom DeLay have recently said that friends within the FBI acknowledged that the bureau is ready to indict Clinton and are making sure they have more than enough evidence to do so.

"I have friends that are in the FBI, and they tell me they're ready to indict," DeLay, a Texas Republican, told Newsmax's "The Steve Malzberg Show." "They're ready to recommend an indictment and they also say that if the attorney general does not indict, they're going public."