Mystery surrounded the disappearance of Russian presidential hopeful Ivan Rybkin (search) on Monday, with police and campaign staffers saying they still had no leads about the missing candidate, a Kremlin critic with close ties to a fierce foe of president Vladimir Putin.

Rybkin's wife filed a missing persons report on Sunday, saying no one had heard from her husband since Thursday evening.

His disappearance comes just five weeks before the March 14 presidential election, which Putin is expected to win easily.

Poll results suggested Rybkin would draw less than 1 percent of the vote, but he promised to be one of the few candidates who would campaign aggressively against Putin.

He launched his campaign last week with a stinging attack on the president, whom he accused of basing his power on blood, citing the war in Chechnya. Putin's tough attitude toward Chechen rebels boosted his popularity ahead of his election in 2000.

Rybkin is a member of Liberal Russia (search), a party supported by Boris Berezovsky (search), a billionaire who was a powerful Kremlin insider in the Yeltsin years but who fell out with Putin and was granted asylum in Britain.

In the last 18 months, two lawmakers from Liberal Russia have been gunned down in murky circumstances.

Rybkin, a national security chief under former President Boris Yeltsin (search), failed to show up for a news conference Friday, missed his formal registration as a candidate Saturday and hasn't answered his cell phone since Thursday night, said Alexander Tukayev, deputy chairman of Rybkin's Liberal Russia party. He has also not contacted his wife, close friends or campaign staff.

Police searched Rybkin's apartment, office, garage and cottage on Sunday but said they found no signs of foul play or clues to his whereabouts. No ransom demands have been made, police said.

Gennady Gudkov, a Russian lawmaker who claims to have close ties with security officials, was quoted by Russian news agencies as saying Monday that Rybkin was found alive and well in a resort outside of Moscow. Police said they could not confirm the claims.

Rybkin's driver dropped him off Thursday evening outside his Moscow home, Tukayev said. A campaign worker spoke with him briefly later that night, but by 10 p.m., he was no longer answering his cell phone.

When Rybkin's wife returned home at 11 p.m., she found the house empty. But the mail had been brought inside and Rybkin's jacket was in one of the rooms, Kommersant newspaper quoted Albina Rybkina as saying.

"I think that when he was at home, someone called him and asked for a quick meeting," she was quoted as saying. "Clearly, they proposed that the meeting would be short because my husband didn't call me or leave a note."

In April, Liberal Russia lawmaker Sergei Yushenkov (search) was shot outside his apartment building in Moscow. Prosecutors have accused Mikhail Kodanyov (search), the chairman of a rival branch of Liberal Russia, of organizing the killing together with five accomplices as part of a power struggle. The trial of some of the suspects is currently underway in Moscow.

Another co-chairman of Liberal Russia, Vladimir Golovlyov (search), was shot and killed in August 2002. Some have suggested that his murder was politically motivated, but others link it with his alleged involvement in privatization fraud in the metals industry, which is believed to be riddled with criminals.

Russian media have said that Rybkin was not known to have any unusual business dealings and that no direct threats had been made against him.

On Monday, prosecutors in a Moscow district opened a murder investigation into Rybkin's disappearance, saying it was standard procedure. But hours later, city prosecutors closed the investigation, saying "there is no basis to open the case."