While Hillary Clinton and her aides have repeatedly characterized the FBI probe as a security review, it's unclear whether the probe is still so limited or is now a criminal investigation. | Getty FBI: Clinton email probe 'ongoing'

The FBI's investigation into Hillary Clinton's email server remained "ongoing" as of earlier this month, a top FBI official said in a letter filed in federal court Monday.

In the new letter, made public hours before a key test in Clinton's bid for the Democratic presidential nomination, FBI General Counsel James Baker was tight-lipped about the inquiry but told a senior State Department official that divulging details about the probe could undermine it.


"We remain unable to provide the requested information without adversely affecting on-going law enforcement efforts," Baker wrote to State Department acting legal adviser Mary McLeod. The letter, dated Feb. 2, was filed by the Justice Department Monday in connection with a Freedom of Information Act lawsuit.

The fact that the FBI was investigating Clinton's server set-up became public last July. A Justice Department official confirmed the inquiry, but said it was a security review of a potential breach of classified information and was not a criminal investigation targeting Clinton, as the New York Times initially reported intelligence and State Department oversight officials had sought.

Since then details about the focus and status of the probe have been scarce, although the FBI has interviewed some former State Department officials about their email practices, including former Secretary of State Colin Powell.

FBI Director James Comey has pledged that the review would be free of political influence.

Last month, White House spokesman Josh Earnest said it appeared Clinton was not a target of the FBI probe, although he seemed to have been referring back to the initial Justice Department statement last summer and did not have any updated information on the status of the inquiry.

While Clinton and her aides have repeatedly characterized the FBI probe as a security review, it's unclear whether the probe is still so limited or is now a criminal investigation.

Baker's letter doesn't appear to address that issue directly. He notes that in September he declined to confirm or deny any inquiry, but now says other statements by the FBI have made clear a probe was going on.

"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. The FBI has not, however, publicly acknowledged the specific focus, scope or potential targets of any such proceedings," Baker wrote. "Thus, while the FBI's response to you has changed to some degree due to these intervening events, we remain unable to provide the requested information without adversely affecting on-going law enforcement efforts."

A Clinton campaign spokesman did not immediately respond to a request for comment on the new FBI letter, which emerged just before New Hampshire voters are set to cast ballots in the first in the nation presidential primary. But during Thursday's debate there, Clinton said she was "100 percent confident" nothing would come of the FBI probe, which she continued to describe as a “security review.”

Clinton’s Republican rivals for the White House have been regularly suggesting in recent weeks that she will be indicted for her use of a private email server that the State Department now says contained at least 22 messages deemed “Top Secret” and about 1,600 classified at lower levels.

“This is getting more and more serious,” Sen. Ted Cruz (R-Texas) said in Iowa late last month.

CORRECTION: This story has been corrected to note that the July New York Times report was about a "criminal referral" from the Intelligence Community and State Department inspectors general. It was later made clear it was a "security referral," and did not request a criminal investigation.