A top aide to Hillary Clinton "refused" to cooperate with investigators looking into whether the Clinton team suppressed a 2012 records request that could have exposed their private email use years before a separate congressional inquiry did so, Sen. Chuck Grassley said Monday.

The Senate Judiciary Committee chairman raised concerns about the role Cheryl Mills, then-chief of staff to Clinton, played in blocking a Freedom of Information Act request for Clinton's email use in a pair of letters to the State Department and the agency's inspector general Monday.

Grassley cited an inspector general report made public in January that detailed, among other things, the extent of officials' knowledge of Clinton's private email use throughout the State Department. Dozens of agency staff were aware Clinton used private addresses to shield her communications.

But Mills intervened when a watchdog group, Citizens for Responsibility and Ethics in Washington, submitted a FOIA request for records "sufficient to show the number of email accounts of, or associated with, Secretary Hillary Rodham Clinton."

"Ms. Mills and senior department officials knew about Secretary Clinton's use of private email for official correspondence since they were sending emails to her non-government email address," Grassley wrote in his letters Monday. "They would have known instantly of records responsive to that request."

"Yet, it was approximately five months later before the department officially responded to CREW's request for email accounts associated with Secretary Clinton," the Iowa Republican continued. "And its response was misleading, at best: 'no records responsive to your request were located.' "

However, more recent FOIA requests over Clinton's emails have forced the release of thousands of responsive records, including the 55,000 pages of her emails presently published or under review by the State Department.

Grassley said Mills "refused to speak with" officials in the State Department's inspector general office when they approached her about her involvement in smothering CREW's records request in 2012.

According to the inspector general, a State Department spokesman had flagged the potentially damaging FOIA request for Mills, who passed it on to a trusted staffer and White House liaison and instructed her to find out how the agency planned to handle the request. Months later, the watchdog group received a notice that the records they sought did not exist.

Grassley demanded to know why the State Department waited so long to respond to such a simple request, and why the eventual response failed to turn up documents that clearly existed.

The State Department has come under fire over the past year for its handling of FOIA requests, which more than doubled after the discovery of Clinton's private email use in March of last year.

Agency officials have battled dozens of FOIA lawsuits in court, only to have judges shoot down State's arguments against releasing Clinton's records. A handful of such federal cases has forced the agency to hand over documents related to her diplomatic tenure.