Syrian rebels trained by U.S. military said they turned over equipment and vehicles to militants linked with al-Qaeda to ensure safe passage through a dangerous area and were threatened with an ambush if they did not do so, according to the U.S. military.

The explanation comes three days after U.S. Central Command (CENTCOM), which oversees military operations in the Middle East, reversed course on a previous denial and acknowledged late Friday that a group of Syrian rebels trained and equipped by the United States had turned over at least a quarter of their equipment to Jabhat al-Nusra, al-Qaeda’s branch in Syria.

It’s the latest embarrassment for the $500-million train and equip program, which was organized to stiffen resistance to the Islamic State militant group. Col. Patrick Ryder, a CENTCOM spokesman, said Monday that the incident occurred Sept. 21, one day after about 70 Syrian rebels who graduated from the train-and-equip program returned to Syria.

About 30 of the graduates rejoined a Syrian opposition group that has been vetted by the United States. The group’s commander, who has not been trained by the United States or coalition forces, planned to move them to another town and was contacted by a militant intermediary. The commander told the U.S. military that he was warned that unless he gave up some of his new equipment, his troops would be ambushed on their way to the new location, Ryder said.

The Syrian rebels turned over six vehicles, believed to be Humvees, to the militants on the way to the town, and then drove back the following day and provided a portion of their ammunition, too, the commander later reported. He contacted the U.S.-led military coalition on Friday.

“We will look at what we can do to prevent such a situation in the future, but given the complexity of the battlefield it is not possible to eliminate all risk,” Ryder said. “We are using all means at our disposal to look into what exactly happened and determine the appropriate response.”

The training program has been under close scrutiny for its effectiveness, especially after Gen. Lloyd Austin III, CENTCOM’s commander, testified this month that there were only “four or five” trained Syrian rebels fighting the militants. The military has since said that another 70 rebels have joined them — including the group that turned over equipment to Jabhat al-Nusra.

CENTCOM said in a statement Friday that if the reports of the transfer of equipment was true, it would be “very concerning” and “a violation of the Syria train and equip program guidelines.” The U.S. military has struggled to find recruits for the train and equip program, and to get them through vetting to make sure they do not have ties to militants.

The Daily Beast reported Sunday night that the commander who turned over the equipment to the militants had been rejected by the United States for training. That suggests that those who did graduate from the program then returned to Syria and reported to the rejected commander.

Ryder, in a statement released Monday, said that the coalition does everything it can to screen participants in the program and monitor the equipment they are given.

“These standards are one of the reasons that we have only trained a small number of fighters to date,” Ryder said. “Even with this screening and other precautions, we can’t control every situation that these fighters encounter once they return t‎o Syria.”

The battlefield in Syria, Ryder added, has areas controlled by the Islamic State, he al-Qaeda-linked affiliate and moderate rebels and the terrain can shift quickly.

“It’s important to be clear eyed about the conditions in which these forces operate, the groups working against them, and the need to overcome the challenges they have encountered and will continue to face,” Ryder said.

Missy Ryan contributed to this report.