Terrible day... my uncle was in the area at the time of the march and he said the place was in chaos.
Supposedly rocks were thrown at the British soldiers by some of the marchers (confrontations like that were very common back then) and it appeared that the soldiers opened gunfire on hearing word that an IRA sniper was in the area. Irish & British journalists, local residents and marchers have all insisted that the soldiers fired into and unarmed crowd... and the fact that none of the soldiers sutained any injuries like bullet or nail bomb wounds probably backs that up. I'm not sure if we will ever know what really happened!
I find it really tragic that many of the people killed were either shot in the back as they fled or shot as they aided people who were already wounded... that is the part that doesn't fit with me. Seemed like the soldiers were picking off random people or something... either way, a truly tragic event that was a turning point in the troubles... for the worse.