Russia's Investigative Committee has closed its case into last year's murder of liberal opposition politician and former deputy prime minister Boris Nemtsov.

Five defendants - Zaur Dadaev, Shadid Gubashev, Anzor Gubashev, Temirlan Eskerkhanov and Khamzat Bakhaev - will now stand trial on charges of committing murder for payment, and the illegal acquisition, transportation and storage of firearms, the top law enforcement body said on its website on June 20.

Prosecutors claim that Ruslan Mukhudinov, a low-ranking officer in Chechnya's Sever battalion, masterminded the plot and offered the men RUB15mn ($234,000) for the Kremlin-critical politician's assassination. 

Nemtsov, who served as deputy prime minister from 1997 to 1998 during Boris Yeltsin's presidency, was shot dead in Moscow on February 27, 2015, while crossing a bridge within sight of the Kremlin. President Vladimir Putin reacted by calling the murder a "provocation" and said he would personally oversee the investigation.

Russia's Federal Security Service (FSB) reported on June 20 that Nemtsov was killed with a home-made gun constructed from parts trafficked from abroad.

"One of the problematic issues in the fight against illicit trafficking of weapons is the growth of the illegal market for firearms manufactured from so-called mass-dimensional layouts (MDL) and civilian weapons, component parts of which are identical to their battle analogues," the FSB said, adding that in recent years, the flow of smuggled MDL in Russia from abroad increased, primarily from Ukraine.

Investigators said they have not confirmed that the murder was linked to Nemtsov's earlier statements in support of the controversial Charlie Hebdo cartoons depicting the Prophet Mohammad.

"The collected evidence shows that the participants of the organised group started preparing for Nemtsov’s murder back in September 2014, which fully rules out the motive of murder on grounds of religious hatred that was mentioned," Investigative Committee spokesman Vladimir Markin said.

In the speculation surrounding Nemtsov's death, some linked it directly to his condemnation of Putin's rule, implying if not the direct involvement of state agencies then mavericks supportive of the president in his stand-off with the West over Ukraine.

Nemtov, 55, was also outspoken in his accusations of corruption in his country, telling bne IntelliNews in a 2010 interview that "corruption in Russia is on a par with Africa".

Ilya Yashin, a friend and fellow opposition leader, described the killing to the Associated Press as "an act of terror" and "a political murder aimed at frightening the population".

At the time of his death, Nemtsov was planning an opposition march in the capital against Putin's economic policies and Russia's role in East Ukraine. He was also preparing to publish a book on Russia's activities in the Ukraine conflict, but senior state officials have strenuously dismissed any link to his assassination.

"The hints that Nemtsov's book 'Putin. War' is somehow linked to the murder of this Russian politician, with this tragedy, will remain on the conscience of those who draw these parallels,"  Foreign Minister Sergei Lavrov said last April after talks with his Swiss counterpart Didier Burkhalter.