The Republican chairman of the Senate Judiciary Committee is directly questioning whether Hillary Clinton still maintains her security clearance despite the ongoing FBI investigation into the mishandling of classified information, according to a new letter to Secretary of State John Kerry.

In the Aug. 24 letter, which marks a significant expansion of the Republican senator's inquiry, Sen. Charles Grassley writes, "Since the [intelligence community inspector general] IC IG made a referral to the FBI pursuant to 50 U.S.C. §3381, has Secretary Clinton's security clearance been suspended pending the outcome of the inquiry?”

The suspension of a clearance is standard practice, a kind of interim step taken to preserve security until an investigation is complete, according to current and former military intelligence officials.

"If you have a circle of people and they are the source of the leak, all of them, normally would be suspended from their positions, they can't go in the building," Chris Farell, a former special agent for Army counterintelligence, told Fox News. Farrell now works as an investigator for Judicial Watch, which is suing for Clinton records.

"The clearances would be suspended by the security manager and it would be made known that there is an active investigation into the loss of classified information, and no information should be shared with these individuals pending the outcome," Farrell added.

In the four-page letter, Grassley said, based on a letter from Clinton's personal attorney David Kendall, that neither he nor his associate Katherine M. Turner had sufficient security clearances to handle the emails -- which contained at least two highly classified documents identified by the intelligence community's IG.  

"... neither Mr. Kendall nor Ms. Turner have a security clearance at a sufficient level to be a custodian of TS/SCI (Top Secret/Sensitive Compartmented Information) material," he wrote.

Grassley's letter also for the first time laid out a timeline that is key to the FBI investigation. The senator said Clinton provided copies of the emails to her lawyer in December 2014. Classified intelligence was identified in the emails in May 2015, but it wasn't until July that the State Department, in a highly unusual move, supplied Kendall with a safe -- a further admission that while Clinton says no documents stamped classified were sent or received by her, the State Department was concerned enough to provide secure storage to Kendall.

Grassley wrote: "... in light of that particular classification, which generally requires advanced protocols such as a Sensitive Compartmented Information Facility (SCIF) and other similar arrangements to possess and view, it appears the FBI has also determined that Mr. Kendall and Ms. Turner's clearance was not sufficient to maintain custody of the emails."

The FBI took physical custody of the server and thumb drives earlier this month. The statute covering the unlawful removal and retention of classified information falls under 18 US Code 1924.  There are also specific rules laid out by the Office of the Director of National Intelligence on "denials and revocations" of security clearances and “damage assessments” for the unauthorized release of national security data. 

Fox News asked spokesmen for the Clinton campaign and the State Department whether Clinton maintains an active security clearance -- and if the clearance was active after she left State, whether Clinton was also read onto compartmentalized intelligence, as well as special access programs.

Authorization to access and hold such information would have been necessary given the two TS/SCI emails identified on Clinton’s server.

In an earlier letter to Grassley, Kendall said, "Please be assured that my firm has followed the guidance provided by the Department of State, which even provided physical equipment necessary to safeguard the thumb drive at issue. Subsequent to that guidance and as this matter unfolded, we have received further guidance from the Department of Justice."

A State Department spokesman said, “We do not confirm details of individuals’ security clearance.” There was no immediate response from Clinton’s campaign.

Catherine Herridge is an award-winning Chief Intelligence correspondent for FOX News Channel (FNC) based in Washington, D.C. She covers intelligence, the Justice Department and the Department of Homeland Security. Herridge joined FNC in 1996 as a London-based correspondent.