LONDON  Shortly after the radiation poisoning in London of a former K.G.B. officer, Alexander V. Litvinenko, a senior Russian official asserted that Moscow had been tailing his killers before he died but had been waved off by Britain’s security services, according to a cable in the trove of secret American documents released by WikiLeaks.

The Russian assertion, denied by British officials, seemed to revive a theory that the British intelligence services played a murky role in the killing  a notion voiced at the time by some in Moscow to deflect allegations of the Kremlin’s involvement in the murder.

The cable, dated Dec. 26, 2006, and marked “secret,” was one of several in the WikiLeaks trove that tried to examine the still unanswered question of who exactly ordered the use of a rare radioactive isotope, polonium 210, to poison Mr. Litvinenko, leading to his death on Nov. 23, 2006. Russia produces polonium commercially, but the process is closely guarded and British investigators have concluded that the isotope could not have been easily diverted without high-level intervention.

In a telephone interview, Marina Litvinenko, the widow of the former K.G.B. officer, called the Russian assertion “disinformation.”