How long was the Manhattan project kept a secret?

The Manhattan Project was kept a secret for about three years, from its inception in 1942 until the end of World War II in 1945. The project's details were tightly controlled, with only a limited number of individuals aware of its true purpose: developing atomic weapons. Even after the bombings of Hiroshima and Nagasaki in August 1945, the full extent of the project remained classified for some time. It wasn't until the late 1940s and early 1950s that more comprehensive information about the project was publicly disclosed.