Long an open secret, the U.S. government has for the first time officially acknowledged that nuclear weapons were stored on Okinawa during the Cold War.

In a statement on the Department of Defense’s Open Government website, the Pentagon revealed Friday “that U.S. nuclear weapons were deployed on Okinawa prior to Okinawa’s reversion to Japan on May 15, 1972.”

The Defense Department statement also acknowledged “that the U.S. government conducted internal discussion and discussions with Japanese government officials regarding the possible re-introduction of nuclear weapons onto Okinawa in the event of an emergency or crisis situation.”

Although widely known — various accounts and documents of a secret deal had previously shed light on the storage of atomic weapons on the islands both before and after Okinawa’s reversion — the issue had been controversial because Japanese leaders and U.S. officials had consistently denied the presence of such weapons within Japanese territory.

In a 1967 address to the Diet, Prime Minister Eisuke Sato introduced the nation’s Three Non-Nuclear Principles, which, reflecting public sentiment, have guided the country’s nuclear policy since.

The three principles, which helped Sato win the Nobel Peace Prize in 1974, stated that Japan shall neither possess nor manufacture nuclear weapons, nor shall it permit their introduction into Japanese territory.

The National Security Archive at George Washington University said in a posting Friday that “however welcome the release may be, its significance is somewhat tempered by the astonishing fact that U.S. Air Force photographs of nuclear weapons on the island have been publicly available for over 25 years.”

The nongovernmental group posted several photos, which were originally released in 1990 from U.S. Air Force collections at the National Archives and Records Administration. The photos had apparently gone unnoticed until now.

Okinawa remains home to a large number of U.S. military bases, and has been the site of recent protests over the transfer of U.S. Marine Corps Air Station Futenma, which has dominated headlines over the past year.

Japan is the only nation to have been attacked with nuclear weapons. The atomic bombs dropped on Hiroshima and Nagasaki in 1945 killed an estimated 200,000 people and, ultimately, led to the Japan’s surrender in World War II.