Four suspects in the high-profile murder case of opposition politician Boris Nemtsov have been detained, according to the Federal Security Service (FSB) and the Russian republic of Ingushetia’s Security Council.

According to FSB chief Aleksandr Bortnikov, the authorities originally detained two suspects on Saturday, who were identified as Anzor Gubashev and Zaur Dadaev.

Bortnikov said both suspects come from Russia’s southern region of the North Caucasus, a restive place with insurgency and crime problems.

The Investigative Committee confirmed that the two detainees are implicated in both organizing and executing the hit on Nemtsov. “We are continuing our work to establish which individuals may be involved in this crime,” the committee’s spokesman Vladimir Markin said.

Later on Saturday, the secretary of the Security Council of Russia's Republic of Ingushetia told Sputnik that two more people have been apprehended by the authorities in connection with the investigation.

Dadaev used to serve as deputy regiment commander in one of the Chechen Republic’s interior ministry units, reported RIA Novosti citing Albert Barakhoev, chair of the security council of Ingushetia, another Russian southern Republic bordering Chechnya. Gubashev worked as a private security contractor in Moscow.

According to Barakhoev, Dadaev was detained in the Ingush capital of Magas, while Gubashev was intercepted en route to Voznesenovskaya, an Ingush village where his mother lives. The Gubashevs were born in Chechnya, but the family moved to Ingushetia back in the 1960s, he said.

Both Gubashev and Dadaev own apartments in Grozny, the Chechen capital, he added.

The two other detainees are Gubashev’s younger brother and a man suspected of being the driver of the car used by Nemtsov’s killer to escape the crime scene.

READ MORE: Opposition politician Boris Nemtsov killed in the center of Moscow

Nemstov, a former Russian governor and deputy prime minister, who became a prominent opposition figure in the early 2000s, was gunned down in central Moscow on February 27. The assassination triggered worldwide condemnation and calls for swift investigation.

The politician was killed on a bridge over the Moscow River near the Kremlin as he was returning home with a female companion. The shooter hit Nemtsov in the back four times and fled in a getaway car.

The crime is being investigated by a joint task force, which includes the police, the FSB and the Investigative Committee.

Investigators are looking into five possible motives behind Nemtsov’s assassination. According to Vladimir Markin, spokesman for the Investigative Committee, the murder may have been a provocation to destabilize the political situation in Russia.

It could also be linked to threats Nemtsov received over his stance on the Charlie Hebdo attack in Paris, or the current war in Ukraine. The politician’s business activities and a possible assault related to his personal life are also being looked into.

READ MORE: Crowds of mourners pay respect to slain opposition leader Nemtsov

The prime witness to the crime, Ukrainian model, Anna Duritskaya, who was accompanying Nemtsov, has since returned to Kiev. She told the media she was unable to identify the killer.

The killing happened two days ahead of an opposition rally Nemtsov helped organize. The rally was replaced with a mourning march in central Moscow, which drew tens of thousands of people.