Russian President Vladimir Putin asked law enforcement entities Wednesday to do more to secure Russia against high-profile political murders like the killing of oppositionist leader Boris Nemtsov last Friday.

“Russia should be made secure at last from the disgrace and tragedies of the kind we all experienced and witnessed just recently. I am referring to the daring killing of Boris Nemtsov in the very heart of our city,” Putin said, according to the Russian news agency Itar-Tass.

Shortly after Nemtsov’s murder, the Kremlin called it a “provocation” meant to discredit Putin and his government. Nemtsov was a prominent opposition leader in Russia and one of only a handful of Russian politicians to openly accuse Putin’s government of directly intervening in eastern Ukraine on behalf of pro-Russian separatists there. He was preparing a report he said would prove Russia’s involvement in Ukraine and had an anti-war march planned for just two days after his murder, which largely turned into a memorial following his death.

Nemtsov, who was Russia’s first deputy prime minister under former President Boris Yeltsin, was particularly critical of Putin, who Yeltsin tapped to succeed him in 1999. Some of Nemtsov’s allies believe someone in the Kremlin played a role in Nemtsov’s murder, although the Kremlin denies the allegations. Critics of Putin alluded to a number of other suspicious murders of oppositionists over the last 15 years as evidence. Nemtsov himself had told the media he suspected that Putin would try to kill him for his opposition to the conflict in Ukraine.

U.S. President Barack Obama drew criticism from Russian Foreign Minister Sergey Lavrov when he said Nemtsov’s murder was “a symptom of [a] climate … in which civil society, independent reporters, citizens, trying to communicate through the Internet, more and more feel they are under threat.”

Russian security services are investigating Nemtsov’s murder with a suspicion it was a contract killing. Alexander Bortnikov, head of Russia’s Federal Security Services, said “there are always some [suspects],” when asked by reporters if anyone had been linked to the killing.