On 4 February -- one day before his disappearance -- Rybkin spoke to RFE/RL's Moscow bureau. During the interview, the outspoken politician said federal security agents had begun following him, even on trips abroad, in what he said was an attempt to intimidate him from taking part in the presidential vote.
"[They are keeping a close watch] on me all the time, to put it mildly, but we will not surrender," Rybkin said.
But the details of Rybkin's disappearance are anything but clear.
The case was opened yesterday by Rybkin's wife, Albina, who said she has not seen or heard from her husband since the night of the 5 February.
But later that day, Boris Berezovskii -- the self-exiled oligarch who is reported to be Rybkin's financial backer -- said Albina Rybkina had told him she had received assurances from the State Duma's security committee that her husband would reappear, unharmed, the following day.
"She was assured there that there is no need to worry. A man on duty in the Security Council told her not to worry. He said there is no need to worry and that Ivan Petrovich [Rybkin] will appear on Monday [9 February]. That's all I know from the official sources," Berezovskii said.
This morning, Moscow prosecutors announced -- and quickly retracted -- an inquiry into whether Rybkin had been murdered.
By afternoon, the ITAR-TASS news agency was citing a member of the State Duma Security Committee as saying Rybkin had been found "safe and sound." But later in the day, that declaration, too, was withdrawn.
Rybkin, himself a former head of the Security Council and onetime Duma speaker under Boris Yeltsin, is the leader of the Liberal Russia Party, which has seen two of its top officials assassinated over the past two years. The party's co-founder and Duma deputy Sergei Yushenkov was shot and killed in broad daylight in April 2003; Vladimir Golovyov, who had been under investigation for corruption, was killed in August 2002.
It is unclear if Rybkin's disappearance is tied to either of those incidents. ITAR-TASS has reported that police have no evidence that Rybkin was abducted or harmed. No one has contacted the Rybkin family or demanded ransom.
Rybkin is a vocal critic of Russian President Vladimir Putin and his policies on issues like Chechnya and the arrest of Yukos oil chief Mikhail Khodorkovskii. He has also accused Putin of amassing control of the country's business sector and has called him "the biggest oligarch in Russia."
Like the other five candidates in the March presidential contest, Rybkin stands little chance of unseating the incumbent. Putin, who enjoys approval ratings of plus-70 percent, is expected to win the race easily.
The mysterious circumstances surrounding Rybkin's disappearance has already sparked several theories about his possible fate, ranging from a Kremlin-ordered political kidnapping to a Berezovskii-orchestrated stunt to boost Rybkin's public standing.
Ksenia Ponomaryova, the head of Rybkin's election headquarters, dismissed the notion the disappearance was a publicity stunt. "If one was to categorize different versions [of what might have happened to Ivan Rybkin], the first one, obviously, is criminal, and the second version, which has been voiced by several politicians and political analysts, is a PR move -- that this is a PR trick by Ivan Petrovich [Rybkin]. And as far as the latter version is concerned, I really have some objections to it, because it's not Ivan Petrovich's style. I've known Ivan Petrovich very well for many years, he's a decent and responsible man," Ponomaryova said.
Igor Bunin, the head of the Center of Political Technologies, a Moscow-based think tank, offered a third possible variant: that Rybkin, having leveled his pointed political accusations against Putin, lost courage and is simply lying low. "Rybkin," he says, "is a civilized and shy person."