Former Secretary of Defense Ash Carter unexpectedly passed away, his family said in a statement on Tuesday. He was 68. 

According to the statement, Carter died Monday in Boston following a “sudden cardiac event.” His family added that Carter had “devoted his professional life to the national security of the United States and teaching students about international affairs … He was a beloved husband, father, mentor, and friend. His sudden loss will be felt by all who knew him.”

Carter served as the 25th U.S. defense secretary from Feb. 2015 to Jan. 2017, and the last under former President Barack Obama. 

He was responsible for a number of successful initiatives during his tenure, with his family noting that he “launched the successful campaign to defeat ISIS in Iraq and Syria, opened all combat positions to women, and forged new connections between the Department of Defense and the nation’s technology community.”

He also enacted a major change within all branches of the military by ending the ban on transgender troops, and changed regulations to allow women to serve in all combat positions. 

Carter served under five presidents during his time at the Pentagon, working in a variety of roles within the nation’s military brass. He also had a distinguished academic career, and was a professor at Harvard University’s Kennedy School of Government. 

Kennedy School Dean Douglas Elmendorf mourned Carter in a statement, and also confirmed he had died from a heart attack.