Natalya Estemirova, who had angered Chechen authorities with her reports of torture, abductions and killings, was found with two close-range gunshot wounds to the head in a wooded area in the province of Ingushetia, The Washington Post quoted police as saying.
Russian President Dmitry Medvedev condemned Estemirova's killing and ordered investigators to "take all necessary measures" to solve the crime more quickly than the killings of other government critics, the Post reported Thursday.
Rachel Denber, deputy director of Human Rights Watch, called for the Russian president to do more.
"What we really need is a truly independent, comprehensive and transparent investigation," she told the Post. "We need Medvedev to go further and say that impunity for these kinds of crimes in Chechnya is rampant."
In a statement from U.S. National Security Council spokesman Mike Hammer, the White House urged Russia to "bring to justice those responsible for this outrageous crime and demonstrate that lawlessness and impunity will not be tolerated."
The Post quoted witnesses as saying four men had seized Estemirova Wednesday outside her home in Grozny, the Chechen capital, and forced her into a car. Police said they discovered her body eight hours later, nearly 50 miles away.
Estemirova, 50, had for more than a decade documented human-rights abuses for newspapers and a rights organization. Citing colleagues, the Post said she had ignored death threats and that her persistence incensed local leaders, including Chechen President Ramzan Kadyrov, who has been accused of participating in beatings and torture.