Two men have been detained in Russia in connection with the murder of opposition politician Boris Nemtsov, the country's Federal Security Service, or FSB, has revealed. The men, identified as Anzor Gubashev and Zaur Dadayev, were detained on Saturday, FSB director Alexander Bortnikov said, speaking on Russian television, the BBC reported.

Bortnikov said both suspects come from Russia’s southern region of the North Caucasus, a restive place with insurgency and crime problems, Russia Today reported. The investigation into Nemtsov's death is ongoing, Bortnikov said, adding that Russian President Vladimir Putin had been informed of the arrests. Nemtsov, a former Russian deputy prime minister and ardent critic of Putin, was gunned down last Friday in Moscow, a short distance from the Kremlin.

He had reportedly been preparing a report that provided evidence of the involvement of Russian military forces inside Ukraine, where pro-Russian rebels have been involved in a violent conflict with forces loyal to the pro-Western government in Kiev since April 2014.

Some foreign leaders, including Ukrainian President Petro Poroshenko, have speculated that Moscow was behind Nemtsov's assassination, in a move designed to silence a government critic.

Russian activists close to Nemtsov have in recent days said that the report he was working on will be published at an unspecified future date, despite Russian security services seizing the slain politician's laptop, which was believed to contain the only copy of much of the evidence he had obtained.

Other prominent critics of the Russian government have also been assassinated in mysterious circumstances in recent years. Journalist Anna Politkovskaya, a prominent critic of Putin, was gunned down in her Moscow apartment in 2006. Five men have been convicted of her murder by contract killing, but those responsible for dispatching the killers have never been discovered.

In addition, former KGB (the former name of the FSB) agent Alexander Litvinenko was killed in 2006, after two Russian men that he met with in London poisoned him with a radioactive substance. A public enquiry into his death has been ongoing in the U.K. since January 2015.