Read to the end. I added more, because my previous response was sparse in facts and detail!

I looked up facts that the Wikipedia page cited, and found the following:

1. Kurusu (special envoy sent with Ambassador Nomura) was reportedly moved under heavy police guard to Hot Springs, Virginia. This fact, reported in Wikipedia, can be confirmed by viewing this image of the Bismarck Tribune that they cite, which provides more basic details.

2. As the article points out, all diplomatic staff were made to wait there as their departure to Japan was arranged, according to that.

3. I have managed to verify that the M.S Gripsholm was indeed a prisoner exchange ship, and civilian exchange ship, used in WWII.^^1

4. I haven't found anything to say definitively that Kurusu was sent back on the Gripsholm, but will do more searching later.

Hope that gives you an idea!

Edit: More searching done, and I just want to clarify that both Kurusu and Nomura were regarded as ambassadors or having the capacity of ambassadors, in most places I've seen. Their full names, for reference are Kichisaburō Nomura and Saburō Kurusu.

This unreliable-looking source says Nomura was aboard the M.S Gripsholm as well. However, I've been unable to find any other information besides a book that I can't seem to get access to. At this point, I think it's fairly safe to say that Nomura and Kurusu, along with staff, were sent back to Japan in 1942 after being held while the arrangements were made. Also, according to one source with the Red Cross who was aboard the M.S Gripsholm, the Japanese passengers were "in excellent physical condition" ^^2, meaning that it doesn't appear they were treated harshly at all.

This may be a tad more than you're asking for, but I did look up what happened after the war as well.

The Lewiston Daily Sun reported this weird thing where the title said they would face trial, but the article itself said they wouldn't face trial. However, I did more digging, and this copy of the Niagara Falls Gazette (warning, PDF format) says that they were going to escape trial as war criminals. Also, glancing at the list of those tried in the Tokyo Trials confirms that they were not tried there. There were other trials, but I haven't found out if they were tried at all besides the above.

Nomura died at 86, seemingly not in prison and of illness, in 1964.

This book confirms the repatriation and that he was apparently not imprisoned after the war, and went on to serve in the Japanese government.

This newspaper article from the Spokane Daily Chronicles confirms that Kurusu was never tried for war crimes, and wanted to live a private life, and died at 68.

^^1 The Red Cross and War Prisoners Social Service Review, Vol. 16, No. 4 (Dec., 1942), p. 678 Published by: The University of Chicago Press

^^2 Red Cross Nursing Service and the "Gripsholm" Ella Gimmestad The American Journal of Nursing, Vol. 44, No. 1 (Jan., 1944), pp. 31-32

Please note that, in AskHistorians, the onus is on you to provide appropriate evidence for the historical claims you make. Stating that it can be found on Google is not helpful, nor is pointing to a Wikipedia page. Mentioning something you may have heard once is not helpful. Guessing based on what you remember from X number of years after the event is not helpful.

Proper comments for this subreddit should be in-depth, comprehensive, and informative while properly situating the answer in its historical context. It should also be backed by appropriate primary and secondary sources, whether or not explicitly mentioned in the comment (i.e. you should always be ready to provide sources if asked). In other words, answers here should be such as an historian of the subject would give.

Thank you.

On a related note, what happened to the American amabssador in Japan?

what happened to the emperor? was he put to death? or just imprisoned. where did he serve his time?

I can give some insight on the short-term answer to this question, what the Japanese ambassador and his staff were up to during the attack and later that same day.

The Japanese embassy was not notified about the attack beforehand, but they received a coded message in the early morning of December 7th formally breaking diplomatic relations with the United States (not actually declaring war). The note was intended to be delivered before the attack but didn't make it to the American government on time for several reasons. The note was very long and the embassy only had one decoding machine. The section instructing the Japanese embassy to deliver the note at 1:00 PM Washington time was at the very end of the message, so the embassy staff didn't realize its importance until it was too late. On top of that the Japanese embassy staff were apparently poor typists, and they were under instruction not to employ any American secretaries for assistance. The note didn't make it to Secretary of State Cordell Hull's office until 2:20 PM, an hour after the attack had begun.

The Japanese ambassador Kichisaburo Nomura and special envoy Saburo Kurusu arrived at the State Department office in Washington at 2:05 PM and were received by Secretary of State Hull 15 minutes later. Hull read the document and commented on it to the Japanese diplomats, describing it as "...crowded with infamous falsehoods and distortions". The Japanese diplomats then left without making any comment.

All of this information comes from Pearl Harbor and the Coming of the Pacific War by Akira Iriye, mostly from P.95-98

Kichisaburo Nomura and the Japanese 'special envoy' Saburo Kurusu were interned on the east coast following the bombing of Pearl Harbor while both the Japanese and American governments negotiated for the exchange of their diplomatic personnel and citizens. After the war neither were charged with any crimes, and both held firm in their claim that they were completely unaware of the plan to attack the US. Cordell Hull seemed to believe Nomura.