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MOSCOW — The man convicted of shooting dead a prominent Russian opposition leader just a few hundred yards from the Kremlin has been sentenced on Thursday to 20 years in prison for the murder.

Prosecutors were dissatisfied because they wanted Zaur Dadayev to be sentenced to life for the contract-style killing of Boris Nemtsov in 2015.

They said they would study the judge's decision before deciding whether to appeal.

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Zaur Dadayev attends the sentencing at the Moscow District Military Court on Thursday. VASILY MAXIMOV / AFP - Getty Images

Nemtsov's former aide, Ilya Yashun, told state-run news agency TASS: "Of course, it's not enough. What is 20 years for a human life?"

The shooting, carried out on a bridge in central Moscow, sent shock waves through the country's already beleaguered opposition.

Four other people convicted of involvement in the shooting were sentenced between 11 and 19 years Thursday. Investigators said they are still gathering evidence in an attempt to discover who organized the murder.

Nemtsov was a top opponent of Russian President Vladimir Putin.

At the time of his death, he was working on a report about Russia's military involvement in the war in eastern Ukraine and in the annexation of Crimea. The report was released three months later.

Dadayev, the convicted shooter, was an officer in the security forces of Chechnya's Kremlin-backed leader Ramzan Kadyrov.

Nemtsov's allies have criticized investigators for not considering a possible role of Kadyrov himself in the killing.

Russian opposition leader Boris Nemtsov attends an opposition rally in center of Moscow in October 2013. Kommersant Photo / Getty Images file

Mansur Mirovalev reported from Moscow. Alexander Smith reported from London.