If they were believing Trump, then they were believing the election was stolen and that Trump had actually won. We can characterize it as they suspected massive fraud, but it's still based on Trump's lie. Trump never had any evidence to substantiate his claim to begin with.
To be clear, it's just one man, Trump, that wanted to overturn a fair election. There was no way the attack on the capitol was ever going to change the results of the election.
The attack on the capitol was spearheaded by small groups of radicals, some of whom are known white nationalists, and who were the one's bent on violence. I imagine that most of the people just followed their lead and got caught up in the moment.
We're going to get two sides from the media, one downplaying the incident by calling it protesting, and the other side calling it what it was, an insurrection, a violent uprising against an authority or government.