MOSCOW, Russia (CNN) -- The head of a leading Russian human rights group accused the presidents of Russia and Chechnya of complicity in murdering their top activist in Chechnya.



Natalya Estemirova, pictured in 2007, had been openly critical of Chechnya's president, Ramzan Kadyrov.



"I am confident about who killed Natalya Estemirova. We all know this person. His name is Ramzan Kadyrov, President of the Chechen republic," Oleg Orlov, the chairman of the Russian human rights group Memorial said on the group's Web site Thursday.



"Ramzan was intimidating and insulting Natalya, and considered her his personal enemy. We don't know whether it was him personally who ordered her [murder] or it were his aides who wanted to please their boss. As far as [Russian] President [Dmitry] Medvedev is concerned, it seems that he doesn't mind having a murderer as head of one of the Russian regions."



Estemirova, 50, was kidnapped outside her home in Chechnya Wednesday, Orlov said, citing eyewitnesses, and found dead in the neighboring Russian republic of Ingushetia later the same day.



Her body was riddled with bullets, Russian prosecutors said -- several shots to the abdomen, and one to the head.



The winner of three international human rights awards, she was a leading activist in the North Caucasus area who had been openly critical of Kadyrov and his methods.



Kadyrov vowed Thursday that he would personally oversee the investigation and assure her killers were punished.



"She couldn't have had enemies among reasonable people," he said in a statement on the Chechen government Web site. "Those who took away her life have no right to be called humans, they don't deserve any mercy, and should be punished as the most cruel criminals.



"I have no doubts whatsoever that those who ordered and conducted this crime will face trial," he said. "That is the very least that the government and society must do in commemoration of Estemirova."



Medvedev said Thursday the murder was "a very sad event," adding it was "absolutely clear... her murder is linked to her professional activities.



"Her type of professional activity is needed by any normal state. She did very useful things. She spoke the truth. She openly, sometimes harshly assessed some types of processes that occurred in our country, and this is the value of human rights workers, even if they are not comfortable," Medvedev said Thursday on a visit to Germany.



His host, German Chancellor Angela Merkel, warned the killing could affect Russian-German relations.



"This is an unacceptable event, and especially if we want to intensify the relations between our two countries, it cannot remain unsolved," she said in a joint appearance with Medvedev.



Medvedev had earlier condemned the murder and said her killers should punished to the full extent of the law, his office said.



Estemirova shouted that she was being kidnapped, as she was forced into a white Lada automobile that had stopped on the road in front of her house on Wednesday morning, Orlov said.



An unidentified man grabbed her and shoved her into the car, Orlov told CNN.



Estemirova had spent years investigating human rights abuses in Chechnya, the restive Russian republic where Russian forces have fought two conflicts against separatists since 1991. Russia officially ended its military mission in Chechnya in April of this year.



The activist told CNN in 2007 she was investigating dozens of abductions and murders that had become the norm in Chechnya, where security forces were fighting a dirty war against separatist rebels.



She joins a growing list of journalists and activists killed after criticizing the Russian authorities, many of whose murders remain officially unsolved.



Top United States and European officials condemned the murder of Estemirova and demanded that the killers be brought to justice.



"Such a heinous crime sends a chilling signal to Russian civil society and the international community and illustrates the tragic deterioration of security and the rule of law in the North Caucasus over the last several months," U.S. National Security Council spokesman Mike Hammer said in a statement.



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