WASHINGTON (CNN) — Al Qaeda in the Arabian Peninsula represents a “serious threat” to attack the United States, according to a Defense Department official who oversees special operations.

In testimony before a Senate Armed Services subcommittee, Michael Sheehan, the assistant secretary of Defense for special operations/low-intensity conflict, said the United States has made important gains against the al Qaeda affiliate over the past year, but “the group’s intent to conduct a terrorist attack in the United States continue to represent a serious threat.”

The threat from al Qaeda in the Arabian Peninsula remains in spite of the death of Anwar al-Awlaki, the American-born radical cleric who became the public face of the group.

Al-Awlaki had been tied to the attempt to blow up a U.S. commercial airliner as it approached Detroit on Christmas Day in 2009 and to the cargo plane bomb plot the next year. He was killed by a CIA drone missile attack in September.

There are still key players at large in Yemen: AQAP leader Naser al-Wuhayshi, a close associate of Osama bin Laden, and Ibrahim al-Asiri, the skilled bomb-maker believed to be behind the aircraft bombing plots; as well as a number of former Guantanamo Bay detainees.

Sheehan also testified about bin Laden’s core al Qaeda based in Pakistan. “We have made progress on this front, but al Qaeda is a highly adaptive organization and we must continue to work with Pakistan to address threats emanating from this region.”