In the latter case, prosecutors in the regional capital had "opened a criminal case," said Chernyshova.
Last week, Kasyanov delivered box loads of signatures, more than 2.2 million according to his supporters, to the election commission in support of his registration for the election.
Even if he is registered, analysts see little chance of electoral success for Kasyanov.
The Kremlin has thrown its full weight behind Dmitry Medvedev, Russia's first deputy prime minister and the preferred choice of Vladimir Putin, the Russian president.
On Tuesday, Nikolai Konkin, an election commission official, said that after checking 400,000 of Kasyanov's signatures, some 62,000, or 15.57 per cent, had been judged inadmissible.
Electoral law allows a maximum of five per cent of signatures to be judged invalid.
On Saturday, Kasyanov had complained that Russian law enforcement agencies had been hounding him and activists of his People's Democratic Union who were responsible for gathering the signatures.
Yelena Dikun, a spokeswoman for Kasyanov, dismissed the prosecutor's claims as "political pressure".
Kasyanov said in comments published by the newspaper Nezavisimaya Gazeta on Tuesday that his campaign was needed "to ensure the country does not slip into a totalitarian dead end".
The requirement for two million signatures applies to Kasyanov but not to Medvedev because Kasyanov does not represent a party in the Russian parliament.
Yevgeny Volk, a political analyst who heads the Moscow office of the US Heritage Foundation, a conservative think tank, said it was quite possible the authorities would remove Kasyanov from the contest.
He said the presence on ballot papers of two other prominent politicians who have a record of loyalty to the Kremlin, in addition to Medvedev, would be used to give the election an air of legitimacy.
Volk said: "The Kremlin isn't interested in Kasyanov participating in the elections and creating opposition to Medvedev. The scenario of his removal from the elections is highly plausible.
"If he was loyal, the prosecutor's office would close its eyes to violations" in the signature lists.
Russia's election system was heavily criticised by parliamentarians from the Organisation for Security and Cooperation in Europe, which often acts as an election watchdog, after legislative polls last month.