Stanislav Markelov was shot dead on Monday afternoon in broad daylight in downtown Moscow. Novaya Gazeta journalist Anastasia Baburova, 25, was wounded in the shooting and died in hospital later on Monday.
"I will sign an order setting up an investigative team, and we will bring in the best officers from the Moscow Criminal Investigation Department," said Vladimir Pronin.
He said an identikit image issued of the suspect was based on CCTV camera footage as no eyewitnesses have come forward.
"It is a young male, some 180 cm (5.9 feet) tall, of slim build," he said, adding that the killer's image was blurred, which made it almost impossible to obtain more information on the suspect's feature.
Markelov had protested the early release of former Russian army colonel Yury Budanov.
Budanov, who commanded a tank regiment during the second military campaign in Chechnya, was convicted in the summer of 2003 of strangling 18-year-old Elsa Kungayeva three years earlier and was sentenced to 10 years in jail. He was paroled earlier this month.
Besides representing Kungayeva's family, Markelov had also started an independent investigation into a near-fatal attack on Mikhail Beketov, editor-in-chief of the local Khimki Pravda newspaper, who fought a campaign to protect a forest from destruction by real estate developers.
The Czech Republic, which currently holds the EU presidency, also called on the Russian authorities "to investigate Markelov's murder, as well as previous attacks on human rights activists, journalists and members of non-governmental organizations, and to bring those guilty to justice."