MOSCOW, RUSSIA - MARCH 29: Russian opposition politician Boris Nemtsov looks on in memory of the victims of a blast inside the Lubyanka metro station on March 29, 2010 in Moscow, Russia.  At least 38 people were killed and 60 injured as two separate female suicide bombers blew themselves up on trains on Moscow's metro during morning rush hour. Twenty-five people died in the Lubyanka station blast and around 45 minutes later a second explosion occurred at the Park Kultury station leaving another 12 people dead.  (Photo by Dmitry Korotayev/Epsilon/Getty Images) *** Local Caption *** Boris Nemtsov
Dmitry Korotayev/Epsilon/Getty Images
MOSCOW, RUSSIA - MARCH 29: Russian opposition politician Boris Nemtsov looks on in memory of the victims of a blast inside the Lubyanka metro station on March 29, 2010 in Moscow, Russia. At least 38 people were killed and 60 injured as two separate female suicide bombers blew themselves up on trains on Moscow's metro during morning rush hour. Twenty-five people died in the Lubyanka station blast and around 45 minutes later a second explosion occurred at the Park Kultury station leaving another 12 people dead. (Photo by Dmitry Korotayev/Epsilon/Getty Images) *** Local Caption *** Boris Nemtsov
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(CNN) —  

A Russian military court has sentenced five Chechen men found guilty in the killing of opposition leader Boris Nemtsov to lengthy prison terms, Russian state media agency Tass reported Thursday.

Nemtsov, a vocal Kremlin critic and deputy prime minister in the late 1990s to President Boris Yeltsin, was shot dead in February 2015 as he walked with his girlfriend in central Moscow.

Former Chechen soldier Zaur Dadaev, who investigators say acted as the gunman, was handed a 20-year sentence. Four other Chechens – all of whom were convicted of being accomplices in the same court last month – were handed terms ranging between 11 and 19 years. All five will serve their time in a high-security penal colony.

Zaur Dadayev (C) is seen in court after being handed a sentence of 20 years in a high-security prison colony on Thursday.
Valery Sharifulin/TASS/Valery Sharifulin/TASS
Zaur Dadayev (C) is seen in court after being handed a sentence of 20 years in a high-security prison colony on Thursday.

Zaur Dadayev had initially confessed to his role and that of the others in the crime, according to Natalia Mushnikova, the judge in the case. However, he later retracted his confession after telling human rights activists from Russia’s federal watchdog, the Public Observation Commission, that it was given after being tortured, state news agency RIA Novosti previously reported.

The four accomplices – Anzor and Shadid Gubashev, Tamerlan Eskerkhanov and Khamzat Bakhayev – all proclaimed their innocence throughout the case.

Moscow’s District Military Court also fined each man 100,000 rubles ($1,670). The trial of the five men began in October 2016, seven months after the suspects were arrested.

Related: The complicated life and tragic death of Nemtsov

A Russian court last month found the five men guilty of involvement in Nemtsov's killing.
Ivan Sekretarev/AP
A Russian court last month found the five men guilty of involvement in Nemtsov's killing.

A sixth suspect wanted in connection with the murder blew himself up in a standoff with police in Chechen capital of Grozny, according to reports on state-run television at the time.

Investigators said forensic evidence found in the getaway car and telephone records linked the suspects to the crime. But the lawyer representing the five men told Tass that they will be appealing the verdict “in the coming days.”

Boris Nemtsov speaks during an opposition rally in central Moscow on December 5, 2011.
AFP/Getty Images
Boris Nemtsov speaks during an opposition rally in central Moscow on December 5, 2011.

Russian authorities also suspect another former Chechen military officer of being the mastermind behind the assassination. Ruslan Mukhudinov has been on an international wanted list since November 2015 and has been charged in absentia, according to Tass. A criminal case against him is being investigated separately, Tass said.

Nemtsov’s family said they “will continue to seek legal action against the organizers and the individuals” in the targeted killing.

The family is unhappy with Dadaev’s 20-year sentence, according to one of their lawyers, who criticized the decision not to hand him the maximum sentence of life imprisonment, Tass reported.

CNN’s Emma Burrows and Elizabeth Roberts contributed to this report.