The UN investigators also completed a site visit to what was believed to be the impact zone of the rockets carrying the poison. Technical and military analysis of the impact zone, combined with subsequent laboratory tests, confirmed that the rockets and rocket fragments contained Sarin gas. This analysis of the impact zone and the weapons provided what is called inferential evidence because the results allowed investigators to infer that the attack was purposeful, since it would be unlikely for surface-to-surface missiles loaded with Sarin gas to launch themselves. Witness testimony confirming that shelling took place immediately before the victims started showing symptoms of poisoning further corroborates the conclusion that surface-to-surface rockets were used to deliver the gas. The next step is to prove responsibility by uncovering evidence that links the perpetrator to the crime. This is called linkage evidence. Without linking the crime to a person, we cannot secure accountability. In this case, initial investigations suggested that the type of rockets and launchers used in the attacks on Ghouta were weapon systems known and documented to be in the possession of, and used only by, Syrian government armed forces. If proven, this fact will likely serve as linkage evidence connecting the attack to the Syrian regime. For criminal accountability, it will be necessary to go a step further and identify particular individuals who ordered, carried out, assisted, or tolerated the attack. TAKE HOME POINTS First, investigators, analysts, and lawyers prove their cases by piecing together different sources of evidence for different purposes, allowing them to tell the full story of what happened. In this case, the UN investigators were able to conclude, on the basis of clear and convincing evidence, that chemical weapons were used against civilians. They began with the videos and then used witness testimony, medical exams, medical lab results, weapons analysis, and technical assessments to confirm that the attack was purposeful. Second, as a frontline documenter, remember that footage documenting the commission of crimes is valuable. But your efforts to capture linkage and notice evidence will likely prove to be of greater importance for long-term justice and accountability. With only 7.5 hours on the ground, combined with follow-up analysis, the UN investigation team was able to prove a crime — a chemical weapons attack against civilians — was committed, by clear and convincing evidence. This was the easy part. Proving who did it and how is much harder, and at the time of writing still had not been accomplished. 50 VIDEO AS EVIDENCE: ALL ABOUT EVIDENCE V 1.0 LAW To strengthen the conclusion that civilians were exposed to a nerve agent, the videos were considered alongside clinical medical examinations showing typical symptoms of exposure to a nerve agent, as well as testimony from survivors, nurses, and doctors, detailing the symptoms they saw and experienced after the shelling. The videos, medical exams, and witness testimony are referred to as corroborative evidence, because they back up the results of the blood and urine tests. Together, these sources of evidence confirm exposure to Sarin gas, but again, the evidence does not yet establish whether the poisoning was purposeful nor provide any insight into the source of the gas.