Secretary of State Rex Tillerson's team called on Russia to fully investigate the death of an opposition leader following the conviction of five men.

The men were convicted of killing Russian opposition leader Boris Nemtsov, who was murdered while writing a report that accused Putin of invading Ukraine in order to boost his domestic popularity. Nemtsov's family has accused the Russian government of stonewalling efforts to find out who ordered and paid for the killing. Putin's team has denied any wrongdoing, but the State Department subtly sided with the regime's critics.

"We welcome the news that five of the persons responsible for the murder of Boris Nemtsov have been convicted of the crime," State Department spokeswoman Heather Nauert said Friday. "We call once more on the Russian government to ensure that all involved in the killing of Boris Nemtsov, including anyone involved in organizing or ordering the crime, are brought to justice."

Kremlin-run media reported that Nemtsov's killers, a group of Chechens, said they were motivated by a desire to avenge "negative comments on Muslims and Islam" that Nemtsov had made. But they were paid about $250,000 for the killing and the family believes the architects of the plan are tied to Putin.

"Clearly, investigators and the court did not strive to establish the truth," Zhanna Nemtsova, the Putin critic's daughter, said following the verdict, per Reuters. "It was of course not a proper investigation, but only an imitation of one."

Putin's spokesman affirmed his desire to find the mastermind behind the killing, but cast doubt on whether investigators would ever succeed.

"The president has repeatedly said that such cases are very hard to investigate, but this does not mean that someone could one time give up the search for the criminals. Sometimes such processes take years," Putin spokesman Dmitry Peskov said, according to RT, a Kremlin-backed media outlet. "Of course, the inevitability of punishment must be observed, but objectively we must agree that these cases are extremely hard."