I've been thinking if itachi knew tobi was at the uchiha massacre why did he not inform danzo and the elders. Perhaps its because they already knew he was there and have known about him for a long time.

I think perhaps it was tobi/obito/madara who manipulated the massacre of the uchiha's by manipulating danzo and the elders as part of his plan to get to this stage. I think may have convinced them the uchihas were planning a rebellion when they probably never were

I think it would've been beneficial to tobi to massacre them because not only did it allow him to manipulate sasuke and itachi but it also got rid of the powerful uchiha clan who most likely would've opposed tobi in this war and would've been the biggest threat in stopping his plans if you think of all the powerful uchihas there might have been. It could also have been done as an act of revenge to the clan who rejected him.

whether this prediction is true or not should be revealed when sasuke talks to the one who knows most.