WASHINGTON—Judicial Watch made public Monday a 2012 email chain showing multiple senior State Department executives used then-Secretary of State Hillary Clinton’s unsecured private email to discuss the most sensitive details of the Sept. 11, 2012, terrorist attack on the U.S. Consulate in Benghazi, Libya.

Four Americans, including Ambassador Christopher Stevens, died in the assault, which within hours was attributed by the Obama White House to an internet video that critically portrayed Islam and its founder, Mohammed.

Judicial Watch first sought the emails released Monday in a 2014 Freedom of Information Act (FOIA) request, but they were not released to the non-profit government watchdog until earlier this month by a federal court and only after the group threatened to expand its litigation in the case against the State Department.

It was this FOIA litigation that led to the public disclosure of Clinton’s use of the private email system, according to Judicial Watch.

Hundreds of other State Department and White House documents sought in the 2014 suit were previously released but only after years of litigation and discovery, which continues and may soon include deposing Clinton and Cheryl Mills, her former chief of staff.

U.S. District Court Judge Royce C. Lamberth, who has heard much of the Judicial Watch litigation seeking the documents, has called Clinton’s private email system “one of the gravest modern offenses against government transparency.”