I've always heard that the Manhattan Project employed over 100,000 people in some way, so since this is such a large project, why was Truman never informed, until he became president?
There were 3.5 million government employees in 1945 so that is only about 2.8% of the work force when he took office. Plus there was a war going on and the whole economy had be redirected to a war economy. So the activities of this top secret project was easy enough to hide during such conditions.