WASHINGTON — In one of the longest-held secrets of the Cold War, the Army explored the potential for using radioactive poisons to assassinate “important individuals” such as military or civilian leaders, according to newly declassified documents obtained by the Associated Press.

Approved at the highest levels of the Army in 1948, the effort was a well-hidden part of the military’s pursuit of a “new concept of warfare” using radioactive materials from atomic bomb-making to contaminate swaths of enemy land or to target military bases, factories, or troop formations.

Military historians who have researched the broader radiological warfare program said in interviews that they had never before seen evidence that it included pursuit of an assassination weapon. Targeting public figures in such attacks is not unheard of; just last year, an unknown assailant used a tiny amount of radioactive polonium-210 to kill a Kremlin critic, Alexander Litvinenko, in London.

No targeted individuals are mentioned in references to the assassination weapon in the government documents declassified in response to a Freedom of Information Act request filed by the AP in 1995.

The decades-old records were released recently to the AP, heavily censored by the government to remove specifics about radiological warfare agents and other details. The censorship reflects concern that the potential for using radioactive poisons as a weapon is more than a historic footnote; it is believed to be sought by present-day terrorists bent on attacking American targets.