The United States government made extreme, top-secret plans for preserving national security in the event of a nuclear attack during the Cold War, according to documents recently released to transparency journalism organization MuckRock.

The FBI created "Plan C" in 1956, the same year the Red Army shut down the Hungarian Revolution, and if activated, Plan C would have put the nation under martial law and allowed for nearly 13,000 people to be detained for having links to "subversive organizations."

The FBI held a briefing with three top officials from every department to distribute papers outlining Plan C, which was later given to FBI field offices. The plan was ordered to be destroyed in July 1957.

Although the plan was never released, MuckRock's memos from closed meetings with high-ranking officials give readers an idea of what could have happened. Plan C would have gone into effect "after a war has begun in which the U.S. is involved or may become involved and prior to an actual attack on the U.S. itself," according to the documents.

Government organizations would decide which of their employees were essential and have them hide out in backup offices. Soviet embassies would be searched for weapons, radios and explosives. Up to 900 Soviet diplomats and couriers would be taken into custody.

In an emergency, the government would also have activated a detention program that would have led to arrests for "individuals whose affiliations with subversive organizations are so pronounced that their continued liberty in the event of national emergency would present a serious threat," according to the documents. The exact number of people who would be affected was set at 12,949.

FBI workers were allowed tell their families about Plan C in vague terms only but were also encouraged to make preparations.

"The stressing to all employees that it is the employee's moral responsibility to plan for survival and for the care of his family, to see that his family is aware of such planning, and to go into such planning, in detail, with his family," one memo reads.

MuckRock has requested about 150 additional pages regarding Plan C under the Freedom of Information Act. The FBI is consulting with other agencies before releasing the documents. Plans A and B did not exist.