I think it's important to point out that you can never prove that something is safe or that something will never happen. We're subject to the evidence base we have at this time. The IARC committee looked at that evidence. Basically, there were studies that found effects and studies that didn't find effects. Many animal and in vitro studies were looked at. Based on this examination, they made a recommendation that it be classified 2B, as possibly carcinogenic to humans. That recommendation acknowledged that there was some credible evidence suggesting that there might be a risk in the long term, but that it was impossible to make a causal association at this time.
On March 24th, 2015. See this statement in context.