Mr Nemtsov, a leading critic of Putin, was reportedly hit by four bullets from a passing car as he walked on a bridge, hours after urging Russians to attend a protest rally and march against the "mad, aggressive and deadly policy of war against Ukraine".
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Boris Nemtsov
Marina Litvinenko described news of the veteran liberal politician's shooting as "absolutely devastating" on Sunday, before thousands gathered in Moscow to mourn his killing.
Marina Litvinenko outside the inquiry into her husband's death
She was asked on Radio 4's Broadcasting House programme whether she believed the shooting of Mr Nemtsov near the Kremlin in Moscow was linked to her husband's poisoning.
She replied: "It's a different way to kill a person, but a way to present that anybody who will try to say something against us will be killed."
Thousands march to protest Boris Nemtsov's killing
Thousands protest Boris Nemtsov's murder
(01 of08)
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People with Russian national flags march in memory of opposition leader Boris Nemtsov, who was gunned down on Friday, Feb. 27, 2015 near the Kremlin, in Moscow, Russia, Sunday, March 1, 2015. Thousands converged Sunday in central Moscow to mourn veteran liberal politician Boris Nemtsov, whose killing on the streets of the capital has shaken Russia's beleaguered opposition. They carried flowers, portraits and white signs that said 'I am not afraid'. (AP Photo/Denis Tyrin) (credit:ASSOCIATED PRESS)
(02 of08)
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People carry a huge banner reading 'those bullets for everyone of us, heroes never die!' as they march in memory of opposition leader Boris Nemtsov who was gunned down on Friday, Feb. 27, 2015 near the Kremlin, in Moscow, Russia, Sunday, March 1, 2015. (AP Photo/Dmitry Lovetsky) (credit:ASSOCIATED PRESS)
(03 of08)
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People carry Russian national flags as they march with posters reading ' I have no fear, fight!' near to the place where Boris Nemtsov, a charismatic Russian opposition leader and sharp critic of President Vladimir Putin, was gunned down on Friday, near the Kremlin, in Moscow, Russia, Sunday, March 1, 2015. (credit:ASSOCIATED PRESS)
(04 of08)
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A man prepares portraits of opposition leader Boris Nemtsov who was gunned down on Friday, Feb. 27, 2015 near the Kremlin, to carry on the march in Moscow, Russia, Sunday, March 1, 2015. Russian investigators, politicians and political commentators on state television on Saturday covered much ground in looking for the reason Nemtsov was gunned down in the heart of Moscow, but they sidestepped one possibility, that he was murdered for his relentless opposition to Putin. The posters read in Russian 'those bullets for everyone of us, he fought for the freedom of Russia'. (AP Photo/Pavel Golovkin) (credit:ASSOCIATED PRESS)
Lithuania Russia Nemtsov(05 of08)
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A man holds a poster as people gather outside the Russian Embassy to pay their respect to Boris Nemtsov outside the Russian Embassy in Vilnius, Lithuania, Sunday, March 1, 2015. (credit:ASSOCIATED PRESS)
(06 of08)
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People carry a banner reading ' Boris Nemtsov, we will never forget, we will never forgive' as they march in memory of opposition leader Boris Nemtsov in St.Petersburg, Russia, Sunday, March 1, 2015. (credit:ASSOCIATED PRESS)
(07 of08)
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Russia's opposition supporters carry portraits of Kremlin critic Boris Nemtsov. Words under portraits read (L-R) 'Fight', 'These bullets in each of us', 'He fought for our future' and 'Heroes never die'. (credit:SERGEI GAPON via Getty Images)
(08 of08)
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(credit:PETRAS MALUKAS via Getty Images)
When asked what effect the high-profile death would have on democracy in the country, Mrs Litvinenko said former deputy prime minister Mr Nemtsov and other people had started to be described as "enemies of Russia".
She continued: "Particularly after the war with Ukraine, the atmosphere in Russia is very aggressive. When you see images saying he is an enemy of Russia, Russian people will hate him."
Ms Litvinenko agreed with Mr Nemstov's long-time friend, Vladimir Riskov, who said Mr Putin was politically responsible.
She concluded: "All what happened in Russia, it's the responsibility of this government and Mr Putin, he built this country that became centre of huge conflict between Russia and Ukraine, and a big problem with all west, and it's the full responsibility of this person and his government."
Russia's federal investigative agency said it was looking into several possible motives for his killing.
It said it was possible the murder was aimed at destabilising Russia and Nemtsov was a "sacrificial victim for those who do not shun any method for achieving their political goals."
This suggestion echoed comments by Putin's spokesman and other Russian politicians that the attack was a "provocation" against the state.