Russian President Vladimir Putin has described the murder of Russian investigative reporter Anna Politkovskaya as "tragic".
Many believe Politkovskaya's death was a contract killing
In his first response to Saturday's shooting in Moscow, Mr Putin promised a thorough investigation.
He made the comments to US President George Bush during a telephone conversation, a Kremlin spokesman said.
Ms Politkovskaya was a fierce critic of Mr Putin - and in particular his policies in Chechnya.
She had accused Russian security forces there of abusing human rights.
Investigation into motive
Mr Putin said "all the necessary efforts toward an objective investigation into the tragic death of journalist Anna Politkovskaya" would be made.
Russia's top prosecutor is personally leading the official investigation, focusing on her investigative reporting as a possible motive for the crime.
SUSPECTED CONTRACT KILLINGS
October 2006 - campaigning Russian journalist Anna Politkovskaya shot dead in Moscow
Sept 2006 - first deputy chairman of Russia's central bank Andrei Kozlov shot dead in Moscow
Oct 2005 - former bank head Alexander Slesarev gunned down near Moscow
July 2004 - US editor of Forbes' Russian edition Paul Klebnikov shot dead in Moscow
Oct 2002 - Magadan governor Valentin Tsvetkov killed in Moscow
Nov 1998 - liberal MP Galina Starovoitova killed in St Petersburg
March 1995 - leading journalist Vladislav Listyev shot dead in Moscow
Ms Politkovskaya's newspaper, Novaya Gazeta, has offered a $1m (£534,000) reward to solve the crime.
There is a widespread belief in Russia that her murder was a contract killing - one of several such assassinations in recent years.
Chechnya's pro-Moscow leaders have condemned the murder.
Chechen Prime Minister Ramzan Kadyrov said that although he did not share many of her views, he sent his condolences to her friends and family.
Ms Politkovskaya is due to be buried in Moscow on Tuesday.
The BBC's Emma Simpson in Moscow says Ms Politkovskaya's fellow journalists believe her murder has dealt a severe blow to what remains of press freedom in Russia.
She was found shot dead in a lift at her block of flats. Grainy CCTV footage shows a man in a baseball cap following her inside just before the shooting.
Mr Kadyrov said he had been shocked by the murder, which he called "an impediment to freedom of speech".
"Although Politkovskaya's writings about Chechnya were not objective at times, I sincerely feel sorry for the journalist," he said.
Chechen forces loyal to Mr Kadyrov have been accused of widespread human rights abuses in the North Caucasus region and Ms Politkovskaya condemned them in her last published article.
Mr Kadyrov rejected claims of Chechen involvement in the murder as "presumptions not based on serious evidence... and unworthy of the press and politicians".
Chechnya's pro-Moscow President, Alu Alkhanov, also sent condolences, adding: "We had totally different views of the Chechen processes but Politkovskaya cared about the Chechen people."
In its special edition on Monday, Novaya Gazeta pledged on the front page that: "While there is a Novaya Gazeta, her killers will not sleep easy."