"I know, I am sure of it, who is guilty for the murder of Natalia ... His name is Ramzan Kadyrov," Oleg Orlov, the chairman of Memorial, said in a statement posted on the group's website late on Wednesday.
"Ramzan already threatened Natalya, insulted her, considered her a personal enemy.
"We do not know if he gave the order himself or his close associates did so to please their boss."
The RIA Novosti news agency reported that Kadyrov had personally pledged to oversee the investigation into Estemirova's murder.
Single mother
Estemirova's body was found near the city of Nazran shortly after a number of men bundled her into a white vehicle outside her home, human rights activists said.
Madina Khadziyeva, a spokeswoman at the Ingush interior ministry, said that Estemirova had been shot dead.
"The body had two wounds to the head, it was clear she had been murdered in the morning," she said.
Memorial and Human Rights Watch (HRW) had earlier this month issued a report accusing Chechen security forces of punishing families of alleged fighters by burning down their homes.
About 100 mourners gathered to remember Estemirova, a 50-year-old single mother, outside Memorial's office in Grozny on Thursday.
Human rights supporters also gathered in Moscow, the Russian capital, to protest against her murder.
One person held a banner, reading: "Only cowards kill women".
Medvedev tribute
Dmitry Medvedev, the Russian president, said Estemirova "did very useful things".
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| Human rights supporters gathered in Moscow, to protest against Estemirova's murder [AFP] |
"She spoke the truth and openly, sometimes toughly described some processes that happen in this country," he said.
"That is the value of a rights activist. Even if these things are not pleasant and uncomfortable for the authorities."
Neave Barker, Al Jazeera's correspondent in Moscow, said staff at Memorial are "visibly shaken by the news".
"Her colleagues ... are also very bitter as well. They feel she has fallen foul of the leadership in Chechnya," he said.
"The feeling amongst colleagues is that Kadyrov is perhaps directly involved in her death."
'No mercy'
After Russia ended a 10-year "counter-terrorism" operation in Chechnya earlier this year, it handed security responsibilities to Kadyrov.
He has also recently been given licence to target rebel fighters in neighbouring regions.
Russia's Gazeta newspaper, reported on Thursday that Chechen authorities had complained about Estemirova's work.
"By coincidence, just before the murder, Chechnya's rights ombudsman Nurdi Nukhazhiyev called on Memorial's Grozny office chief to complain about Estemirova, saying she refuses to see positive changes and insists on bringing up dirt," it said.
In 2007, Estemirova was awarded the Anna Politkovskaya prize - named after the murdered Russian journalist - by the Nobel Women's Initiative, a group established by female Nobel Peace Prize laureates.
The United States condemned the murder and urged Russia to hunt down those responsible.
"We call upon the Russian government to bring to justice those responsible for this outrageous crime and demonstrate that lawlessness and impunity will not be tolerated," Mike Hammer, the White House National Security Council spokesman, said.