Dimitri Kovtun and Andrei Lugovoi are suspected of murdering the 43-year-old former spy, who died nearly three weeks after consuming tea laced with polonium-210 in London in November 2006.
According to Interfax, which hosted the press conference in Moscow, Kovtun said: “I am more than sure that he dealt with polonium, without knowing it.
“Maybe it was leaking and polonium accumulated in his body gradually. It is possible that something he carried with him led to a gradual accumulation of polonium in the body.”
On his deathbed, Litvinenko accused Russian president Vladimir Putin of ordering his assassination — which the Kremlin denies.
Kovtun and Lugovoi also deny any involvement and remain in Russia, having initially refused to take part in the inquiry in London.
However, last month Kovtun dramatically changed his mind and offered to give evidence via video link.
The inquiry’s chairman Robert Owen said he would grant Kovtun “core participation status” and allow him to give evidence if he meets conditions, including that he provides a full witness statement and discloses any relevant material.
Owen said Kovtun would not be granted access to restricted confidential material despite his core participation status, putting him in the same position as Litvinenko’s widow and the Metropolitan Police.
The inquiry has been adjourned until July 27.