Army soldiers move the flag-draped remains of Army Staff Sgt. Kevin McEnroe, of Tucson, Ariz., from a C-17 aircraft at Dover Air Force Base in Delaware on Nov. 7. (Alex Wong/Getty Images)

The three Army Special Forces soldiers killed at a Jordanian military base this month were working for a CIA program to train moderate Syrian fighters when they were shot at a checkpoint under still-unclear circumstances, U.S. officials said.

The Nov. 4 slayings of the three soldiers is believed to be the deadliest single incident involving a CIA team since December 2009, when seven officers and contractors were killed in a suicide bombing in Khost, Afghanistan.

The Fort Campbell, Ky.-based soldiers — all members of the 5th Special Forces Group — were killed by a Jordanian soldier at an entry control point to Prince Faisal Air Base near Jafr, in the southern desert about 150 miles south of the capital, Amman, according to the officials. The shooter also was wounded in what was described by U.S. and Middle Eastern sources as an exchange of gunfire.

The soldiers, identified as Staff Sgt. Matthew Lewellen, Staff Sgt. Kevin McEnroe and Staff Sgt. James Moriarty, were among roughly 2,000 U.S. troops working in Jordan while participating in the U.S.-led campaign fighting the Islamic State. Some of the troops have been assigned to mobile artillery units along the Jordanian border while others assist CIA-led training programs for Syrian opposition fighters.

The CIA declined to comment on the shooting incident or on the soldiers’ possible role in agency programs.