Mr. Sakhanh was found to have withheld details about time he had spent in Italy, including an arrest in 2012, Inspector Blombergsson said. The ensuing investigation linked Mr. Sakhanh to the video of the killings.

The video, which the rebel who shared it said had been filmed for fund-raising, had not previously been publicly posted. It showed the man identified by the Swedish authorities as Mr. Sakhanh among a group of rebels as their commander, Abdul Samad Issa, recited a sectarian monologue and ordered the killing of seven shirtless and terrified prisoners, five of them bound.

The commander, known as “The Uncle” by his fighters, fired the first shot, into the back of one prisoner’s head. Then the others opened fire. The man identified as Mr. Sakhanh was shown wearing a neat brown jacket and appeared relaxed and joking. He was holding a Kalashnikov rifle, with which he shot a helpless man at his feet.

During the inquiry, Inspector Blombergsson said, the investigators learned that Mr. Sakhanh lived for some time in a town near Milan, and had been arrested in February 2012 after participating in an occupation of the Syrian Embassy in Rome. He fled Italy and joined rebels in Syria, and began working with the commander known as the Uncle.

Fingerprints from his Swedish immigration application matched those from his criminal file in Italy, Inspector Blombergsson said, and in his mug shot from Italy he appears to be wearing the same brown jacket that the gunman wore in the mass killing video. The investigators also found another video of the prisoners, with many rebels posing behind them before the killing. Mr. Sakhanh was visible in that scene, too, the police said.