Mr. Barakat saw documents that identified Syrians providing information to journalists as a top national security threat demanding a lethal response. He said he heard discussions of plans to fabricate evidence of rebels attacking civilians. He quit when he concluded there was no intention of reform, fleeing Syria with USB drives and paper documents bandaged to his torso under his clothes. (He defected several weeks before the journalists were killed.)

Another witness — Anwar Malek, an Algerian member of an Arab League monitoring mission — said he quit the mission after coming to believe that members were acting as agents for the government and removing evidence of government attacks on civilians, including the case of a 5-year-old boy killed by a sniper, from reports.

Mr. Malek described conversations in Homs in January 2012 with a member of the crisis committee, Deputy Defense Minister Assef Shawkat, Mr. Assad’s brother-in-law. He said Mr. Shawkat told him that it was necessary to kill civilians to defeat “the terrorists,” that “he would have been able to destroy Baba Amr in 10 minutes if there were not any video cameras” and that foreign journalists reporting from Baba Amr were “agents” of Israel and other countries, who should be targeted.

“‘For us, these are terrorists,’” Mr. Malek quoted him as saying. “They are targets for our military services and our security forces.”

Mr. Malek added that when Mr. Shawkat learned he had visited the media center, he pressed him to disclose its exact location; Mr. Malek demurred.

Soon after, Mr. Malek concluded that the mission was compromised and resigned. He said he received a death threat over the phone in his Homs hotel room, and his convoy to Damascus was shot at. The government blamed rebels, but Mr. Malek believed the attack was staged by the government.

Less than a month later, Syrian activists brought in Ms. Colvin and a British journalist, Paul Conroy, through a mile-long, four-foot-wide water pipe. Soon after, Ulysses said, an informant told intelligence officials that the journalists were at the activists’ media center and described its rough location.