MOSCOW — Three Russian opposition leaders were questioned Friday for a second time just days after tens of thousands marched in Moscow in the largest protest since President Vladimir Putin returned to the Kremlin.

Ex-deputy prime minister Boris Nemtsov, television host Ksenia Sobchak and liberal activist Ilya Yashin were interrogated again for their role in organizing recent rallies, including a May 6 protest that ended in violent clashes with police.

Putin has taken a tougher approach toward the opposition since he began his third term as president in May.

On Monday, police searched the properties of all three leaders and others. A day later, police brought them in for questioning just an hour before the massive anti-Putin protest in Moscow.

Sobchak tweeted that she was brought into a "strange, damp room" on Friday where she was questioned about the $1.7 million in cash found in her apartment. The wealthy TV host has said previously that she does not trust Russian banks.

Both Sobchak and Yashin were released after several hours of interrogation, but both said it was clear that the investigations were going to continue.

"I still don't understand what this investigation is leading toward," Sobchak said afterward. "The questions are very strange."

Sobchak tweeted that, before her passport with an American visa was returned, the investigator asked her if she knew anything regarding the 1963 assassination of President John F. Kennedy. Her money was not returned.

Until recently, Sobchak had been considered untouchable because of Putin's enduring loyalty to her late father, who as mayor of St. Petersburg in the early 1990s gave Putin his first government job.

Yashin said he could only guess what the proceedings would entail, because Russia's powerful Investigative Committee would not discuss the "protocol" for its probe.

"Throwing out a mention of my possible arrest was clearly meant to provoke me to leave the country," Yashin tweeted after his own release. "I don't intend to run away."

Investigative Committee spokesman Vladimir Markin said the investigation into the Russian opposition leaders was ongoing and that his agency was still reviewing all the documents seized in its search.

"The legal status of some of these people may change from witnesses to suspects," he said.

Investigations also continue for seven other opposition leaders and activists, including Left Front leader Sergei Udaltsov and anti-corruption blogger Alexei Navalny.

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  • Alexei Navalny

    <em>Russian opposition leader Alexei Navalny speaks to the media at the headquarters of the Russian Investigation committee in Moscow, Russia, on Tuesday June 12, 2012. (AP Photo/Mikhail Metzel)</em><br><br> The 36-year-old corruption-fighting lawyer and popular blogger has played a key role in mobilizing Russia's young Internet generation to rally against Vladimir Putin's rule. Charismatic and ambitious, Navalny spearheaded a series of rallies in Moscow during the winter that brought up to 100,000 people into the streets in the run-up to the March vote in which Putin won a third presidential term. He reaches tens of thousands through his blog and has more than 250,000 followers on Twitter.<br><br> Navalny has tapped into people's anger over the corruption that pervades public life. After he described Putin's political party as the "party of crooks and thieves," the catch phrase stuck.

  • Sergei Udaltsov

    <em>One of the Russian opposition leaders Sergei Udaltsov, right, greets his supporters after he was released from a detention center in Moscow, Russia, early Thursday, May 19, 2012. (AP Photo/Ivan Sekretarev)</em><br><br> The leader of the Left Front opposition movement, 35-year-old Udaltsov has been at the forefront of the anti-Putin protests for several years. A great-grandson of a Bolshevik revolutionary, Udaltsov has consistently defied the authorities, staging unsanctioned marches and rallies. He also launched numerous hunger strikes and spent weeks in hospitals amid concerns about his health.<br><br> During his political career, Udaltsov has been arrested more than 100 times and spent months in prison.<br><br> Udaltsov refused to show up for questioning on Tuesday and headed right to the opposition march instead.

  • Kseniya Sobchak

    <em>Russian socialite and TV host Kseniya Sobchak, daughter of the late St. Petersburg mayor, Anatoly Sobchak, speaks to journalists during her interview in the Echo Moskvy (Echo of Moscow) radio station in Moscow, Russia, Wednesday, Jan. 18, 2012. (AP Photo/Alexander Zemlianichenko)</em><br><br> The 30-year-old socialite, TV host and restaurateur, who often has been described as a Russian equivalent of Paris Hilton, has become the new glamorous face of the opposition. Sobchak is the daughter of the late mayor of St.Petersburg, who was Putin's mentor in the 1990s.<br><br> A personal relationship with Putin initially seemed to shield Sobchak from reprisals, but that immunity may have come to an end with a raid on her apartment Monday. She tweeted Monday that an investigator told her that she had made a mistake by mixing up with "bad company." `'I never thought that we would slide back to such repressions," she said.

  • Ilya Yashin

    <em>Russian police officers detain opposition leader Ilya Yashin, center, after an unsanctioned protest in Moscow on Monday, March 5, 2012. (AP Photo)</em><br><br> The 28-year-old member of the leadership of the opposition Solidarity movement has been among the organizers of the recent anti-Putin protests. A fiery speaker, Yashin is a passionate critic of the government.<br><br> "A smart government deals with reasons for protest. A stupid government fights protesters," he said on his blog.