Evidence

http://www.trumanlibrary.org/whistlestop/study_collections/bomb/large/documents/index.php?documentdate=1945-07-17&documentid=79&studycollectionid=abomb&pagenumber=1
This link refers to a petition written by the scientists that had been working in the field of atomic power. The scientists felt that the dropping of the atomic bomb could not be justified unless it was made public and Japan was given the chance to surrender. Even then the scientists feel that it is immoral and inhumane. Out of anyone, the scientists working in the field should have the most knowledge of the destruction it can bring.
http://members.peak.org/~danneng/decision/usnews.html
Dr. Leo Szilard was a scientist that worked on the atomic bomb project. However, he was one of the many scientists that grealy opposed the use of it before it was dropped. He talks about the Secretary of War used bad excuses and says that he doesn't think that Truman and all below him thought about other options besides dropping the bomb. He also thinks that other countries may have done the same, but it is still not justified.
http://www.trumanlibrary.org/whistlestop/study_collections/bomb/large/documents/index.php?documentdate=1945-08-11&documentid=11&studycollectionid=abomb&pagenumber=1
Truman sends a letter to Samuel Cavert dicussing Truman's thoughts about dropping the bomb. Truman says he feels the worst about dropping the bomb. His biggest reasons for dropping the bomb were in retaliation of Pearl Harbor and  because the Japenese killed our soldiers in a death march. He thought that the only thing the Japenese understood was when the U.S. would bombard them with bombs. Truman's two reasons were incredibly selfish.
http://www.dannen.com/decision/45-07-04.html
Leo Szilard sent out his petition to all the scientists. He expressed his veiw of since they are the people that worked in the feild, they know best the destruction the bomb could make. He pointed out the moral responsibility that came with using the bomb. He also thinks that the fact none of the American people knew about the choice to drop the bomb increased their responsibility to stop Truman.
http://www.dannen.com/decision/bardmemo.html
Secretary Bard is suggesting that using the bomb without warning Japan is unfair. He descibes America as a country of fairplay and humanitarianism. He says that Japan is looking for a way to surrender but haven't been given the chance to surrender. He is in all, in favor, but two things he said were two large reasons the bombings were not justified.
http://www.doug-long.com/quotes.htm
Admiral Leahy is dissaproving of the usage of the atomic bomb because he thinks that it was no assistance to our war against Japan. He believes that the Japs were already defeated by a strong sea blockade and the, already used, smaller bombings. He know believes that the possibility of future uses with nuclear weapons is scary. He doesn't believe that wars can be won by destroying women and children.