Warning: Video contains blood and disturbing scenes of violence

It’s not clear from the first seconds of the video who is firing or from where, but the rapid fire of the assault rifle is unmistakable. With the shots ringing so clear over the shouts of the crowd, chanting in support of deposed President Mohamed Morsi, so is the proximity. The camera jumps, some protesters step back or turn to run. In only a few shorts seconds – so short, he must have been awfully close – three men carry a fourth back through the crowd, limp and covered in blood.

Then, at 50 seconds into the video, embedded below, the cameraman pans up to see a soldier in military camouflage peering over the top of a building. The soldier aims carefully before each shot – fired, it’s not clear at whom, somewhere into the crowd. More bleeding young men are carried away.

It’s difficult to know for sure the exact provenance and timing of this video, which has circulated widely in and outside of Egypt since military troops opened fire on pro-Morsi protesters this morning, killing at least 40 and injuring 300. It’s purportedly taken from outside Cairo’s Republican Guard headquarters, where Morsi’s supporters believe he is being held and where Monday morning’s violence took place. Separate BBC footage appears to show clashes and a body bag loaded into the back of an ambulance, although it’s not clear if this is from the same incident.

A military spokesman says the troops were defending against armed Morsi supporters; the protesters say soldiers shot at them from rooftops and behind barricades without warning. The video does not definitively prove either of their stories – it is far too narrow and bereft of context to tell us the broader story of what happened – but it is a disturbing moment.

The video was clearly taken during the day; Monday’s shooting mostly occurred during or before dawn. Wall Street Journal report Tamer El-Ghobashy says that several witnesses told him the episode continued until about 8 AM, by which time Cairo is quite sunny.

Washington Post correspondent William Booth visited the Republican Guard headquarters later in the day, where he found protesters and troops still gathered. Here are a few of his photos from the scene, some of which appear consistent with the buildings in the above video: