There is a lot of debate as to Emperor Hirohito's direct involvement in Japan's actions during WWII. It's difficult to say concretely, because different sources will say different things. Some academics in Japan will overwhelmingly say that Emperor Hirohito had no involvement, or that he wasn't aware of what the Japanese military was doing. On the other side of the aisle you have Western scholars who state that Emperor Hirohito was aware of the atrocities and/or was an extremely passive leader who made no inquiries into what was going on and certainly did not stop them. This also gets into the debate about how much power Hirohito actually had, and if he could have actually "overruled" the actions of Japan's military (he probably couldn't have).
However, and this gets to the second part of your question, whether or not the US believed that he played a role in Japan's actions was rendered moot by the fact that the US prosecution's strategy following Japan's surrender was to lay the responsibility of the war and of war crimes on Japan's military leaders such as Tojo, rather than the Emperor. The reasons for this being that the US believed retaining the Emperor as a figurehead would lead to a more peaceful transition for a post-WWII Japan.
Here are some similar questions/answers that you might find interesting: https://www.reddit.com/r/AskHistorians/comments/7vpyl3/hirohito_often_gets_lumped_in_with_hitler_as_an/ https://www.reddit.com/r/AskHistorians/comments/5omexl/how_did_emperor_hirohito_escape_trial_and_death/ https://www.reddit.com/r/AskHistorians/comments/6746xu/what_were_the_thoughts_of_the_japanese_emperor/
More can be written, but you might like to start with a few previous discussions.
For involvement: "Hirohito often gets lumped in with Hitler as an evil dictator in popular parlance, but to what degree can we actually lay the blame for Japanese atrocities like Nanking and Bataan on him?" by Catfulu.
For attempts to prosecute him: "What was Emperor Hirhohito not considered a war criminal once the Allies won?" : [deleted] provided links to answers by /u/phoenixbasileus (who discussed the possibility of indicting Hirohito), /u/tayaravaknin , and /u/TheWinStore .
This is not to discourage discussion. More questions, data, and debate are welcome.