Former Defense Secretary Ash Carter, who opened combat jobs to women and ended a ban on transgender people serving in the military, has died at age 68.

Carter died Monday evening after suffering a heart attack in Boston, his family said in a statement Tuesday.

Known as a defense thinker and strategist, Carter was a nuclear expert, three-time Pentagon executive, budget guru and academician who had served as a defense civilian in the building over a period of 35 years.

Gen. Martin Dempsey, former chairman of the Joint Chiefs of Staff, noted at Carter’s retirement ceremony in 2017 that his focus on the less glamourous aspects of the job such as people management had made him known as the “most important, least known figure in Washington.”