Notorious Russian criminal boss Aslan Usoyan, known as Grandpa Hassan, was shot by a sniper in central Moscow from a sixth-floor stairwell in a neighbouring building, investigators said Thursday.

Investigators were piecing together the dramatic last moments of Usoyan before he was gunned down on Wednesday, as a bystander caught in the crossfire lay in hospital in a critical condition.

A sniper fired a shot at Usoyan as he entered a restaurant on a central street accompanied by two bodyguards, wounding him in the neck, Moscow investigators said in a statement.

One of the bodyguards fired back at the sniper with a pistol. They pushed Usoyan into the restaurant but the sniper fired a further five shots through the closed door, seriously wounding a female member of staff.

"Investigators believe that the shot was fired from a stairwell between the fifth and sixth floors" of a neighbouring building, the Investigative Committee said.

This was where investigators found a folding chair and piece of cloth and six cartridge cases. The sniper is thought to have fired an AS VAL machine gun, which is used by Russian special forces.

The 30-year-old woman suffered gunshot wounds to her hip and chest, investigators said. On Thursday she was in intensive care after being operated on and losing 4.5 litres of blood, RIA Novosti reported, citing a medical source.

Usoyan's bodyguards did not wait for an ambulance but gave him first aid and drove him to the Botkinskaya hospital, where he died, investigators said.

Life News website posted investigators' footage of the open window in the stairwell used by the sniper on a street that is one of central Moscow's swankiest residential areas.

A law enforcement source told Life News that a resident remembered two men who had asked to enter the building in search of a non-existent organisation and then "walked up to the landing between the 5th and 6th floors and looked out of the window."

Two residents said that they received mysterious phone calls asking them whether they were interested in selling or renting their apartments, the source told Life News.

The apparent contract killing reflects a turf war between two clans but could be one of the last such killings as the old-style mafia gives way to a new generation of criminal organisations embedded inside the police and local authorities, criminologist Yakov Gilinsky told Kommersant FM radio station.

"I think this is one of the last great showdowns."