But Sergei Kapitonov, a spokesman for the local branch of the Investigative Committee, stressed in a telephone interview that Tyagachyov was not a suspect.
He also denounced media reports that Tyagachyov has disappeared.
"We did not ask the media to help us find anyone," he said. "This is supposed to be only the investigators' job."
He declined to provide details about the case.
Vadim Chechel, director of the Cascade security company, was shot dead in central St. Petersburg in April 2008. Alexander Druzhinin, who was sentenced to 20 years in prison in May 2009 on charges of shooting Chechel, told investigators that Tyagachyov's former adviser Vladimir Kulibaba had ordered the hit, Interfax said.
Kulibaba's lawyer, Yury Novolodsky, said he believed Chechel had been romantically involved with a former acquaintance of Tyagachyov and that was why investigators wanted to question him.
"As far as I understand, Chechel planned to get married to Maria Stroganova and that's why Tyagachyov is being brought as a witness," Novolodsky said by telephone.
His client, Kulibaba, 41, was detained in November 2010 with another suspect, and he remains in custody, Interfax reported.
Tyagachyov's assistant, Lidia Voytenko, said her boss was out of the country but planned to meet with St. Petersburg investigators next Wednesday.
She also said she had spoken with investigators about articles (1, 2, 3) published by Izvestia, Fontanka.ru and Newsru.com that implied that Tyagachyov had disappeared, and she said they had denied releasing the information to the media.
"I have the papers from the investigators asking for Tyagachyov right in front of me, and we just received them on Tuesday," Voytenko said by telephone. "He's not hiding from anyone. I have the facts, and those media outlets published a lie."
She said Tyagachyov, who represents Rostov in the Federation Council, would sue the media outlets for publishing misinformation.
Izvestia's parent company, News Media, said in an e-mailed statement that the newspaper was ready to defend itself in court, adding that it had not been contacted by Tyagachyov's spokesmen or received any notice from court.
Kapitonov, of the Investigative Committee, said investigators had sent letters to Izvestia and Fontanka.ru, which was the first to publish an article about the matter on Tuesday, inquiring about their sources.