This morning I was looking at a piece of footage posted on Youtube which seemed to prove that the Iranian government is sending Revolutionary Guards to fight in Syria. The camera hovered over five youngish men, several of them bearded. “My team acted in support of the Syrian security forces in the suppression and shooting of Syrian civilians,” said one, who identifies himself as “Sajad Amirian, member of the armed forces of the Islamic Republic of Iran”. He said he and his companions received their orders from the Homs area airforce, and they showed passports which had exit stamps from Iran  but no entry stamps into Syria, suggesting they had not arrived through normal routes.

The men had been captured by the Homs branch of the Free Syrian Army, a group of defecting soldiers who are fighting President Assad’s troops. Rumours of Iranian snipers providing back up to government forces have been circulating for months, so maybe this was the evidence we needed.

Verifying Youtube footage is a complex business. An organisation called Storyful, based in Dublin, has established itself to unpick the clues – who uploaded it? from where? have they uploaded video before? When journalists can’t get into places, and have to rely on video which emerges, such technical expertise becomes an important reporting tool. While they did that, I asked an Iranian journalist not just to translate what the men were saying, but analyse how they said it.

Half way through the morning I noticed that Anita McNaught, a journalist with Al Jazeera, had tweeted out: “Looking for some opinions on this video. What do Farsi speakers think?” A few minutes later she posted an article from December 29th with a photograph of five Iranian engineers who had disappeared in Homs in December. Sure enough, they’re the same guys.

Now, were they really engineers or were they in Homs for another purpose? I have no way of knowing. Mahmoud Haj Hamad, who audited spending in the Syrian Defence Ministry until he fled to Egypt last month told Channel 4 News today that he oversaw the payment of Iranian snipers who would come as “military advisors” for six months at a time. But the footage doesn’t prove it.

More worryingly, it suggests that the Free Syrian Army interrogated these Iranians under duress and tried to use their “confessions” as black propaganda. Which goes to show how careful we have to be before airing footage we didn’t shoot ourselves, and how cruel and dirty this conflict has become.