In the days after this shooting, the Nigerian military posted a video of Donald Trump implied that American troops at the Mexico border should use gunfire to respond to migrants who protest by throwing rocks. That drew international attention to what happened in Abuja.

We worked with researchers at Storyful to see what footage was posted to social media from Abuja. We quickly discovered shocking scenes. It was clear the military overreacted and fired at people as they retreated, but the scenes were chaotic and it was difficult to understand what exactly happened.

We contacted sources in Abuja who sent us more footage. We were able to verify a large number of casualties from the shooting, far more than the Nigerian military admitted. So we dispatched our freelance reporter Emmanuel Akinwotu to Abuja over several days. He met survivors in hospitals, talked to witnesses and collected a trove of video evidence from their cellphones. Much of it had not been seen before.

In New York, we reconstructed what happened step-by-step, putting order to the chaos. We geo-located the site where the shooting happened – a military checkpoint outside Abuja. My colleague Christoph Koettl, a geo-spatial analyst, examined old satellite footage to see where the roadblock is usually set up — on a side road which is parallel to the highway. Emmanuel went to the scene and confirmed through local sources that traffic usually flows freely through here, as it was on the day of the march (the military said the marchers blocked traffic, but in fact the military did, when they slid their roadblock across the highway).

Christoph researched the military from logos and insignia in the footage, he identified the battalion involved in the shooting – the Guards Brigade, an elite unit that has a short chain of command to President Buhari. He also identified some of the weapons they used, including a heavy 50 caliber machine gun we could see and hear in the footage. It blew limbs off some people.

We talked to former U.S. Marines on staff, including our weapons expert John Ismay, about this and other machine guns we saw in the footage. Dionne Searcey, our West Africa Bureau Chief reported out other aspects of the declining security situation in Nigeria, and the history of impunity among the Nigerian military that allows events like this to go unpunished.

Christoph also retrieved old satellite imagery of a mass grave from a 2015 incident – he had reported on that incident for Amnesty International, where he worked at the time. Emmanuel interviewed military spokesman John Agim about our findings. He insisted that the checkpoint was attacked, the marchers intended to occupy Abuja and they had to be stopped. But what the Nigerian military did in this instance – to police unarmed protesters with live gunfire when there was no risk to life – is illegal under international law.