A former Guantanamo Bay detainee was among those killed in recent U.S. airstrikes on terror targets in Yemen, Capt. Jeff Davis said on Monday.

Yasir al-Silmi, who was held at Guantanamo Bay from 2002-2009, was killed in airstrikes on March 2, Davis said.

Davis, a Pentagon spokesman, would not get into specifics about why al-Silmi was released in 2009, but confirmed he was counted among those who had returned to terrorism.

Al-Silmi also operated under the alias Mohammed Tahar.

There have been more than 40 airstrikes in Yemen in the past five nights.

As the Obama administration wound down, officials stepped up efforts to shrink the prison population at Guantanamo Bay, though former President Barack Obama was never able to realize his campaign pledge of closing the U.S. detention facility.

While Obama assured the U.S. in December that only "low-level" terrorist operatives had been released from Guantanamo Bay, the emergence of former detainees taking on high-level roles in terror groups has undermined that message. One, Ibrahim al Qosi, became the face of Al Qaeda in Yemen.

The Pentagon has estimated that at least 30 percent of transferred Guantanamo Bay detainees had returned to the battlefield, killing Americans and U.S. allies. Critics contend the number is far higher than 30 percent.

During its peak, Guantanamo Bay held 778 suspected terror operatives.