Marina Litvinenko spoke at an inquiry into the death of the ex-intelligence officer, who fled to London in 2000. He died in November 2006 after drinking tea laced with radioactive polonium-210 at a London hotel.
On his deathbed, Alexander Litvinenko accused Putin, the Russian president, of ordering his killing.
British officials have also accused the Russian state of being involved in the killing and British police have identified two Russian men as the prime suspects. They deny guilt and Moscow refuses to extradite them.
He interviewed a 17-year-old Chechen who told him his whole class had enlisted to fight Russia.
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"For Sasha, this was shocking," she said referring to her husband by a diminutive. It was "the first time he realized something (was) wrong in this war."
She said his concerns peaked when it was suggested that he kill Boris Berezovsky, a Russian oligarch who had fallen out with the Kremlin.
Alexander Litvinenko was arrested after accusing the FSB of plotting to kill Berezovsky at a 1998 news conference. He was released in late 1999 and fled Russia the following year.
Marina Litvinenko said the family settled in Britain after the US refused to grant them asylum.