Of the 15 federal agencies that receive the most Freedom of Information Act (FOIA) requests, the State Department was determined to be the worst performing, according to a new study from the Center for Effective Government.

The State Department received an overall score of 37 percent - an "F" - significantly lower than the next worst performing agency, the Department of Health and Human Services, which received a score of 57 percent.

The State Department's "particularly dismal" handling of FOIA requests "is completely out of line with any other agencies performance," the report said.

The Center for Effective Government looked at three performance areas: whether the agency had clear rules governing the release of FOIA information as required by law, the quality and ease-of-use of an agencies FOIA website, and the ability of the agency to complete the processing of FOIA requests in a timely manner.

No agency received an "A" from the center, and only two agencies got a "B" - the Department of Agriculture, at 85 percent, and the Social Security Administration, at 82 percent.

The 15 agencies examined by the Center for Effective Government received more than 90 percent of all FOIA requests for each of the last two years.

Noting the State Department's dismal customer service practices, the report found that it was the only agency "in the scorecard whose rules do not require staff to notify requesters when processing is delayed, even though this is mandated by law."

Federal law holds all government agencies to the same FOIA standards, but some agencies are much slower at processing requests.

Despite having 157 full-time employees dedicated to processing FOIA requests - that's 119 cases per staff member for that year - the agency still received the lowest processing score.

The law mandates that agencies respond to FOIA requests within 20 days, but only 8 percent of the State Department's requests were processed within that time frame. The average length of time requester had to wait for appeals was 540 days, the longest of any other agency.

In recent days, the State Department's handling of FOIA requests has come under even more scrutiny due to former Secretary of State Hillary Clinton's sole use of a private email account and home-based server for official government business during her tenure at the department.

Clinton handed more than 50,000 carefully selected emails to the department in December, but those have yet to have been publicly released.

After repeated FOIA requests dating back at least five years went unfulfilled by the State Department, the Associated Press sued the department on Wednesday in an attempt to force the release of email correspondence and government documents from Clinton's time at the department.

The AP is seeking material related to Clinton's public and private calendars, correspondence with aides expected to be involved with her likely presidential campaign and emails related to the Osama bin Laden raid and the National Security Agency's' spying program, AP said.