Former Russian banker German Gorbuntsov is in a critical condition in hospital after being shot near Canary Wharf, London on Tuesday, sources said.
Scotland Yard detectives are treating the attack in the East London business district as attempted murder.
The perpetrator, who was armed with a submachine gun, is unknown. Gorbuntsov, who is in his forties and currently lives in London, owned several banks in Moldova and Russia.
He is believed to have been approached outside his flat in Byng Street on the Isle of Dogs when the attacker struck.
The victim has been put into a medically induced coma. His condition is described as stable.
The banker is under armed guard in hospital while officers from the force's Specialist Crime Directorate, which tackles serious and organised crime, made contact with counterparts overseas, it is understood.
Officers declined to reveal where Mr Gorbuntsov is being treated.
Police said the suspected gunman, who is white, 6ft and slim and was wearing dark clothes, was seen running away from Westferry Road.
Scotland Yard said the shooting was not thought to be gang-related or linked to any other incidents in Tower Hamlets, and there have been no arrests.
Reported by RIA News, Gorbuntsov’s lawyer, Vadim Vedenin, has linked the attack to "his client’s role in a 2009 investigation into a murder attempt on another Russian banker Alexander Antonov".
However, Scotland Yard said it was "too early to speculate" on reports that link the attack to Antonov.
A spokesman added: "This is sitting with Trident which investigates gang-related shootings. There is no reason it should rest with anyone else."
The victim is also wanted in Moldova on charges of embezzlement, however Gorbuntsov’s Moldovan lawyer Valery Andronik said that any assassination attempt on Gorbuntsov is unlikely to have originated in Moldova.
According to RIA, the victim told Andronik "several times" that if he went back to Russia, "they will kill me".
The apparent attempted assassination will prompt comparisons with the murder of Russian dissident Alexander Litvinenko in London in November 2006.
British prosecutors have named fellow ex-KGB agent Andrei Lugovoy as the main suspect in his poisoning with radioactive polonium-210, but the Russian authorities have repeatedly refused to send him to face trial in the UK.
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The man who attempted to murder Gorbuntsov "was known" to his victim, according to his neighbour.
The 54-year-old management consultant was at home when the shooting took place.
He said: "I arrived home at 6.30pm and went upstairs, and around 9pm that night the police knocked on the door and mentioned there had been an incident, and that someone was critically injured and taken to hospital.
"They mentioned that the attacker was known to the person who'd been attacked.
"The morning after, I came down to go to work just before 7am and the glass here in the door was all smashed in.
"When the police mentioned the incident they hadn't said it was physically in the foyer," added the man, who gave his name only as John.
Tony Smith, 26, an estate agent, said he had been trying to drive down Byng Street moments after the shooting.
He said: "I pulled my car up and looked down and saw his body, saw him. Police were surrounding him but there were only two police cars, and the rest came later.
"The window was shot out on the door. I looked at him, I thought he had been stabbed at first. I didn't expect this to happen in this block."
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