Nemtsov, one of the most energetic and vehement critics of the Russian leader, was gunned down on a bridge near the Kremlin just before midnight on February 27, 2015.
The scene of his crumpled body lying with brightly lit Kremlin towers in background was a grisly and vivid symbol of the dangers facing Russian opposition figures.
"As far as (Putin's) personal responsibility is concerned, I think that he is responsible for how the investigation is being conducted, because he said that he will take it under personal control, that he will control it.
The suspected triggerman was an officer in the security forces of Kremlin-backed Chechen leader Ramzan Kadyrov. Four other Chechens also face trial in the case.
But who ordered the killing remains unknown.
"The investigation is very ineffective," Nemtsova said. "It's going on but it's ineffective in terms of organizers and those who ordered this crime. I believe this is politically motivated."
Putin said at a December news conference that he always had a good relationship with Nemtsov and that it is up to the investigators to find out why he was murdered.
Nemtsova echoed the argument frequently made by critics that even if Putin had no direct hand in the killing, he bears a sort of responsibility by encouraging a culture deeply intolerant of opposition, in which critics are often characterised as paid agents in thrall to Western governments.
Nemtsov supporters and mourners plan to hold a memorial march tomorrow. City authorities denied them permission to march to the spot on the bridge where he died.
Many are expected to visit the informal memorial site there after the march, whose turnout may gauge the relative strength of Russia's opposition, which is beleaguered by official strictures and harassment and nearly ignored by state-controlled television.
"It's a very big loss, because there are very few well-known opposition activists," Zhanna Nemtsova said. "He was one of the most prominent, most courageous, most principled people.
