Russian investigators have filed an indictment against four men accused of involvement in the murder of opposition leader Boris Nemtsov, a brazen killing which sent shockwaves among the opposition earlier this year.

Mr Nemtsov, a top opponent of Russian president Vladimir Putin, was shot late at night on February 27 as he was walking across the bridge just outside the Kremlin in Moscow.

Five Chechen men have been arrested on charges of involvement in the killing but it has remained unclear who ordered the attack.

The Investigative Committee, Russia's top investigative body, said in a statement that investigators have filed the final indictment against four of the five suspects and expect to formally indict the fifth one shortly. The committee said the probe is expected to wrap up in January.

Among those indicted was the suspected triggerman, Zaur Dadayev, a former officer in the security forces of Chechnya's regional leader, Ramzan Kadyrov.

A separate probe is expected for the fugitive suspected organiser, Ruslan Mukhudinov, who had worked as a personal driver of senior Chechen police officer Ruslan Geremeyev.

Mr Nemtsov's family has been petitioning investigators to look into Kadyrov's possible involvement in the murder and to question Geremeyev, who is believed to have fled Chechnya.

Mr Nemtsov's family's lawyer, Vadim Prokhorov, told the Interfax news agency that Geremeyev's driver has been singled out as a scapegoat "in order to deflect the attention from Kadyrov's inner circle".