It was ~9pm in December, meaning it was pitch black. The troops that were with him were down in the trench taking cover, with the present officer being the guy helping the king see above it. The only sources I'm aware of that we have from the people who were with him when it happened, is the trench-officer who was there, as well as a lieutenant who was watching the king at the time of his death. Both suggest that the king was hit by a bullet coming from the enemy fortress. Weibull, a 20th century historian in Sweden who was among to first to be critical in his historical research, has commented that:

Indiciekedjan sluter sig. Avgörande bevis kan inte föras. Men uppfattningen, att ingen grund föreligger för antagandet av ett mord på Carl XII, låter sig inte upprätthållas. Vägande grunder för ett dylikt antagande föreligger.

Which roughly translates to:

[...] Concluding evidence cannot be found. But the apprehension that the grounds for the belief that Charles XII was murdered, does not comply. [...]

The main reason, it seems, why this confusion has arisen (already in 1718 when he was shot), is because the person who was to succeed him, Frederick I, was present at that night, and it has been suggested that he had ordered the king murdered. There isn't a shred of evidence for that hypothesis though, and Frederick I didn't actually succeed the king at first either. His wife, Ulrica Eleonora, was made regent queen after the king's death, and it wasn't until two years later that she handed over the rulership to her husband.

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Further reading:
Lauritz Weibulls examination of the event (PDF) (in Swedish)
Liljegren, B. (2012), Karl XII: En biografi