The Columbia Journalism Review recently called NPR social-media strategist Andy Carvin, who's turned his Twitter feed into an invaluable fire hose of on-the-ground updates on the uprisings in the Arab world, a "verification machine" for his efforts to identify the sources behind the information his army of Twitter followers share with him. "Some of these folks are working to actively overthrow their local regimes," he told CJR. "I just have to be aware of that at all times." This morning provided a vivid illustration of the difficulty foreign journalists like Carvin face in sifting through the citizen journalism emerging from the protests in the Middle East and North Africa and sharing that critical information with their readers.

At around 10 a.m. EST, Carvin linked to a graphic video, uploaded on May 8, showing gunmen spraying a man with bullets. The video's title and description suggested in Arabic that the footage showed Syrian security forces firing on a man in the Syrian city of Homs, where the regime recently dispatched tanks as part of an escalated crackdown on protesters. The profile for the user who uploaded the video, bilal1989100, says the user joined YouTube on the day of the upload and hails from Saudi Arabia. In introducing the video, Carvin wrote, "Horrible, graphic video showing dead man getting riddled with bullets from an ak47, reportedly Homs Syria."

But within minutes, Carvin heard from several followers who said the video did not in fact come from Syria. The user @BentBenghazi, a U.S.-based Libyan-American, wrote, "a while back this was said to be in Libya..... Might not be in either place." The Twitter user @AnasAkkawi in Doha wrote, "This video happened in Egypt in Alexandria and not in Syria!!" and added, "I can tell from accent of people shouting it's 100% Egyptian." The user @AhmadBhumi soon joined in: "i see this video in liveleak about a year ago. Mexico Cartel. Allahu Akbar ? they imitating what they hear in iraq war video." @BSyria noted, "this was initially posted by a Syrian activist then removed. he didn't trust its authenticity."

Carvin soon responded, telling @AnasAkkawi "that's why I posted it. People claim it was filmed in Libya as well. And Iraq. And Lebanon. Trying to get to the bottom of it" and @BSyria "Like I said, it's been claimed in half a dozen countries at this point. Should've made it clearer I was trying to ID location." Carvin is still working to uncover the provenance of the video. As he recently tweeted: "So far ppl have credited this execution video to Egypt, Syria, Lebanon, Iraq, Libya & even Mexico. Missing any?"