Board Thread:Naruto Discussions/@comment-26773508-20150905054531/@comment-26275777-20150906233910

Thekillman wrote:
 * The controlling of Kurama like a pet should quickly lead to the conclusion that the man was an Uchiha. The Uchiha weren't allowed near Kurama because of the Sharingan. The Uchiha were believed to be behind the attack even though it was never proven, even though technically it's correct.

I think this is precisely why the Uchiha were suspected; because like you said Hiruzen and team could read the clues and figure out that it had to be an exceptionally powerful Uchiha. Problem: as the leader he already knows what the Uchiha under his command are capable of, and that it can't be any of them (as far as we know, at least). Danzo in particular might have concluded that they were either harboring Madara or more likely were secretly raising someone like him, which is probably why he pushed for the entire clan to be annihilated. In fact, he's entirely unsurprised when "Madara" shows up.