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.

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