For those exact same reasons - if you're a witness to a major event that means your evidence will be required in any court or inquest - it pays to write down everything you've perceived you've witnessed, as soon as possible after the event.
Then the only problem is, whether you saw what you thought you saw.
A year or two after the event, all kinds of other memories intrude on your memory of the event, making many details fuzzy, or just outright wrong.