We did not make any appreciable effort during the war, General Groves explains (in Now It Can Be Told: The Story of the Manhattan Project), to secure information on atomic developments in Japan:
First, and most important, there was not even the remotest possibility that Japan had enough uranium or uranium ore to produce the necessary materials for a nuclear weapon. Also the industrial effort that would be required far exceeded what Japan was capable of. Then, too, discussions with our atomic physicists at Berkeley, who knew the leading Japanese atomic physicists personally, led us to the conclusion that their qualified people were altogether too few in number for them to produce an effective weapon in the foreseeable future. Finally, it would have been extremely difficult for us to secure and to get out of Japan any information of the type we needed.
[…]
Positive support for our reasoning that the Germans were vitally interested in atomic energy had come from Norway, where before the war, in the town of Rjukan, about seventy-five miles west of Oslo, the Norwegians had constructed a complex of hydroelectric and electrochemical plants. When the Nazis occupied the country in 1940, they had required the operators of the Rjukan works to enter into contracts to produce heavy water which was to be shipped to Berlin for experimental use in the development of atomic energy. In September of 1942 we had estimated that approximately 120 kilograms of heavy water were being delivered to the Nazis each month under the terms of this contract.
[…]
The first attempt to put these works out of commission involved the use of guerrilla forces. Some five months after my request, three Norwegians, especially trained in sabotage techniques, and wearing British uniforms, parachuted into Norway, where they were met by local guerrillas. After nearly a week of hard cross-country skiing, they arrived at Rjukan and attacked the factories there on February 27, 1943.
The first reports on this action were most encouraging. A news dispatch from Oslo, which was relayed to Stockholm, stated that damage was “not extensive except at the place where the attempt was made and there the devastation was total.” Subsequent reports from Sweden were even more encouraging, calling this “one of the most important and successful undertakings the Allied saboteurs have carried out as yet during the war.”
These same Swedish newspapers caused me some headaches when they went on to speculate at considerable length about the importance of heavy water, pointing out that “many scientists have pinned their hopes of producing the ‘secret weapon’ upon heavy water, namely an explosive of hitherto unheard-of-violence.” These items were picked up by the London papers and finally, on April 4, 1943, New York readers were greeted by such headlines as “Nazi ‘Heavy Water’ Looms as Weapon.” Immediately, Dr. Harold Urey, who had discovered heavy water, was deluged with calls from reporters wanting more information. He neatly sidestepped all such inquiries with the statement that “So far as I know, heavy water’s uses are confined solely to experimental biology. I have never heard of an industrial application for heavy water, and know of no way it can be used for explosives.”
Meanwhile, the British were hard at work assessing the damage done to the Rjukan works in the February raid. Their first estimates indicated that heavy-water production had been set back by about two years. We had different information, but our suspicions were not confirmed until we learned definitely that the plant had resumed partial operations in April. Yet doubt can be contagious and, under our gentle prodding, Sir John Dill soon felt himself compelled to inform General Marshall that a more realistic appraisal of the damage indicated that the plant could be completely restored in about twelve months. After some discussion of launching another commando raid—a full-scale one this time—General Marshall, at my behest, proposed to Sir John Dill that, instead, the plants be made a first priority bombing objective. This proposal led ultimately to a massive air attack on Rjukan in November of 1943. Although this mission in itself was not particularly destructive, it apparently led the Germans to believe that more attacks would follow. This belief, together with the problem of constant sabotage by workers in the plants, and probably a lack of appreciation at high government levels of the possible value of the product, caused the Nazis to give up their attempts to repair the damage done by the saboteurs in February. All apparatus, catalyzers and concentrates used in the production of heavy water were ordered shipped to Berlin. Norwegian guerrillas interfered with every step of the transfer, successfully destroying much valuable equipment and even going so far as to sink the ferry which carried a large part of the heavy water.
So neither Japan or Germany could have built a bomb, but either could have sown radioactive dust from bombers.