But on Aug. 29, he disappeared from the camp in circumstances that are unclear.

A relative of the colonel from his hometown, Ebleen, in the northwestern province of Idlib, said that he last heard from Colonel Harmoush on Aug. 27, when the colonel called to tell him that he was scheduled to meet with Turkish intelligence officers and to expect good news in two days.

Some Syrian refugees in Turkey, reached by telephone, said that Colonel Harmoush met with Turkish officials in the Hatay refugee camp on that day, and then went with them on an unspecified trip. Others in the camp said that he left the camp to go shopping and never came back.

Image Lt. Col. Hussein Harmoush Credit... Al Arabiya

Human rights activists from Avaaz, a group that helps document protests, said they believed that the Turkish authorities handed Colonel Harmoush back to Syria in exchange for nine members of the Kurdistan Workers Party, an insurgent group that has been fighting for Kurdish autonomy in southeastern Turkey since the early 1980s. The activists offered no proof of their claim.

Another account given by activists and refugees suggested that Colonel Harmoush had been invited to dinner on Aug. 29 along with two other officers who also defected. According to this version, the three Syrians were then drugged and smuggled back across the border to Syria with the help of Turkish intelligence officers. The whereabouts of the other two officers are not known.

None of the accounts could be verified. Turkish officials insisted that they had nothing to do with the affair, and said that a recent investigation had led to the arrest in Turkey of three men, two Syrians and an Iranian, who are believed to be Syrian intelligence operatives.

In an interview, the Turkish foreign minister, Ahmet Davutoglu, angrily denied that Turkey had played any role in Colonel Harmoush’s return to Syria.