Moscow: The murder of Russian opposition politician Boris Nemtsov was carefully planned, investigators said Saturday, pointing to details of the weapon used and the killers' knowledge of his movements.

"There is no doubt that the crime was meticulously planned, as well as the place chosen for the murder," the powerful Investigative Committee, which has been put in charge of the probe, said in its first detailed statement on the drive-by shooting of the 55-year-old politician.

Nemtsov's lawyer Vadim Prokhorov said the politician had received threats on social networks and told police about them, but authorities didn't take any steps to protect him.

The Russian Interior Ministry, which oversees Russia's police force, said that Nemtsov was killed by four shots in the back from a passing car as he was walking over a bridge just outside the Kremlin shortly after midnight.

Interior Ministry spokeswoman Yelena Alexeyeva told reporters that Nemtsov was walking with a female acquaintance, a Ukrainian citizen, when a vehicle drove up and unidentified assailants shot him dead. The woman wasn't hurt and was being questioned by police.

Mikhail Kasyanov, a former Russian prime minister now also in opposition, said he was shocked.

"In the 21st century, a leader of the opposition is being demonstratively shot just outside the walls of the Kremlin!" Kasyanov told reporters as Nemtsov's body placed in a plastic bag was removed on a rainy and cold night, as the Kremlin bells chimed nearby. "The country is rolling into the abyss."