A prominent Chechen, Sulim Yamadayev, was reportedly shot dead in Dubai.
Vladimir Putin's vow to rub out Chechen rebels “in the shithouse” helped to propel him into the presidency.
His role in past surprise results had attracted intense scrutiny and criticism, as well as media allegations of involvement by the Chechen underworld.
Ms Estemirova did not speak Chechen fluently.
Authorities say two police officers and a government worker were killed in the attack in the Chechen capital, Grozny.
Estemirova's husband died during the first Chechen war - at around the time she decided to abandon her job as a history teacher and to embark on a career as an activist and journalist.
Moscow-backed Chechen President Ramzan Kadyrov hasn't exactly gone out of his way to deny responsibility;
Russian army began an attack on Chechnya in October 1999, which Moscow described as an anti-terrorist action. The Chechen rebels have since launched a bloody guerrilla war.
A Chechen group involved in the Beslan school siege in 2004 took responsibility and its leader promised more attacks.
Her father was Chechen, her mother Russian.
In a recent road-rage incident in the Russian capital, two Chechen policemen bearing special security passes beat up and fired at a bus driver who cut in front of their Mercedes.
The Chechen language and culture were suppressed.
A conscript in the first Chechen war (1994-96), Mr Babchenko volunteered to fight in the second (which started in 1999) for reasons he leaves unclear.
A Russian colonel, Yuri Budanov, entered a house in the Chechen village of Tangi, home to an 18-year-old girl, Elza Kungaeva.
