“Investigators are looking into various theories, including that the murder was linked to the victim’s professional activities,” Vladimir I. Markin, a spokesman for the investigative wing of the Prosecutor General’s Office, said of Mr. Markelov.

The murder bore the characteristics of a contract killing, a not-uncommon phenomenon in Russia. Even so, the audacity of Mr. Markelov’s murder surprised some commentators.

“Even when organized crime in the 1990s was rampant, such a killing would have been considered bold and horrific,” said a correspondent from Vesti television.

Mr. Markelov, who was the director of the Rule of Law Institute, a civil liberties group, gained prominence recently representing the family of Elza Kungayeva. She was an 18-year-old Chechen whom Mr. Budanov, the former tank commander, admitted strangling in his quarters in March 2000, just as the second post-Soviet war in Chechnya was beginning to rage.

Mr. Budanov was sentenced to 10 years in prison but was given early parole for good behavior.

Mr. Markelov, at the news conference just before his death, told reporters that he might file an appeal to the European Court of Human Rights against the early release of Mr. Budanov, who was a decorated colonel of the Russian Army before he was stripped of his rank. In an interview last week with The New York Times, Mr. Markelov said he might also file a lawsuit against the administration of the prison that released Mr. Budanov last Thursday.