“Certainly, it is important to find both those who committed and those who ordered this brazen and cynical crime,” Interfax quoted him as saying.

Ilya Yashin, a political ally of Mr. Nemtsov, expressed the widespread skepticism among activists that this case would prove any different from previous murders in which those responsible were never identified. Given the lack of significant details, “it is hard to judge whether these are the real perpetrators or whether the investigation went down the wrong track,” he wrote on his Facebook page.

Mr. Yashin called on the authorities to release any substantial evidence they had, such as pictures from the security cameras, so the public would know that the case was real. More important, he said, the authorities had to identify not just the perpetrators, but whoever was behind them.

It is not uncommon in high-profile cases to detain the perpetrators, which allows the authorities to present a good picture on television, he said. “But if whoever ordered it can avoid responsibility, then the practice of political assassinations will no doubt continue,” he added.

There have been a series of high-profile murders of government critics in Russia over the past two decades in which the mastermind has never been identified. Last June, for example, Moscow’s highest criminal court sentenced five men, all from the North Caucasus, to prison for the 2006 murder of the investigative journalist Anna Politkovskaya. But her supporters stress that the question of who ordered her killing remains open.

Ms. Politkovskaya was a scathing critic of Kremlin policies in the troubled southern Russian republic of Chechnya and of the local strongman, Ramzan A. Kadyrov. The names of the two suspects in the Nemtsov case seem to indicate that they are from Chechnya, where Russia has been fighting insurgents on and off for centuries.

After two bloody wars in the 1990s, which leveled the capital, Grozny, Mr. Putin essentially subcontracted control over Chechnya to Mr. Kadyrov.