Speculation mounted in the Russia press on Tuesday over what had happened to Ivan Rybkin, the presidential candidate and Kremlin critic who has mysteriously vanished five weeks before the election.

"A candidate from among third-rate politicians becomes the main personality in the election campaign," said Nezavisimaya Gazeta, a broadsheet controlled by President Vladimir Putin's arch-foe Boris Berezovky.

"When Ivan Rybkin expressed indignation and made revelations, hardly anyone listened to him. When Rybkin fell silent, this produced the impact of an exploding bomb."

Mr Rybkin went missing near his flat in Moscow last Thursday and news of the 57-year-old's disappearance did not become public until the weekend.

"What was initially a predictable election is becoming increasingly strange and controversial with every passing day," Nezavisimaya Gazeta said.

Election gimmick?

Some asked whether the disappearance of Mr Rybkin, whose campaign is financed by Mr Berezovky, a self-exiled tycoon, was really a publicity stunt.

"Rybkin has moved away," headlined a sceptical Gazeta, a pro-business paper.

"While the search for Rybkin goes on, political scientists are thinking about the reasons for his disappearance - a PR move, a kidnapping?"

"Taking into account the methods usually employed by Boris Berezovky in his political operations of late, this disappearance may be a PR move" to attract attention to the candidate, the paper quoted one analyst as saying.

The popular Moscow daily Moskovskaya Pravda examined potentially murkier sides to the story.

"Fishing in muddy waters", it headlined.

"Rybkin with his tiny rating represented absolutely no danger to the current Russian authorities. But here it is appropriate to recall another circumstance," it said.

Rybkin is no rival to Putin Komsomolskaya Pravda

"Rybkin, who was heavily involved in Chechnya at the end of the 1990s, probably knew many sinister things about relations between the leaders of the Chechen guerrillas and those who inhabited the Russian corridors of power at that time.

"Possibly, the fact that the terrorist act in the Moscow underground and the disappearance of Rybkin occurred at the same time may just be a coincidence. But it cannot be ruled out that these two incidents are somehow linked."

No happy ending

"Finding Ivan Rybkin," was how popular tabloid Komsomolskaya Pravda headlined the story.

"The ending to the story of the disappearance cannot be a good one. If things turn out well... the political image of this partner of Berezovky will have been completely ruined.

"Nobody will believe that he was kidnapped - for example, that Putin's special services frightened him and let him go. Rybkin is no rival to Putin."

Parallels with the recent killings of two other politicians from Rybkin's Liberal Russia party were also drawn in Komsomolskaya Pravda. Sergey Yushenkov was shot dead last April and Vladimir Golovlev was killed in August 2002.

BBC Monitoring, based in Caversham in southern England, selects and translates information from radio, television, press, news agencies and the Internet from 150 countries in more than 70 languages.