A former Russian banker is in a critical condition in hospital after being shot several times in London.
Scotland Yard detectives are treating the attack on German Gorbuntsov near the Canary Wharf business district as attempted murder.
Mr Gorbuntsov was shot as he entered a block of flats in Byng Street on the Isle of Dogs on Tuesday evening.
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.
Mr Gorbuntsov's condition is described as critical but stable.
Scotland Yard said it was "too early to speculate" on Russian reports that the attack may be linked to a 2009 investigation into a murder attempt on another Russian banker, Alexander Antonov, in Moscow.
A spokesman added: "This is sitting with Trident which investigates gang-related shootings. There is no reason it should rest with anyone else."
Mr Gorbuntsov, who is in his 40s, previously owned banks in Russia and Moldova, according to Russian media.
He was returning to his home when the attacker opened fire with a sub-machine gun, the Kommersant newspaper reported.
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. Mr Lugovoy has always denied any invovelement
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.
Mr Litvinenko's widow, Marina, sent her thoughts to Mr Gorbuntsov's family.
"This must obviously be a very difficult time," she said. "I don't think it is very easy to say what has happened at this stage - they will be hoping for some clarity very soon."
Trident officers are treating the incident as a gang-related crime after liaising with the force's counter-terrorism command.
Serviced apartments in the Byng Street area fetch up to £200 a day in rental income.
Homes in the area are the choice of City highflyers, with easy links to Canary Wharf, the O2 Arena and London City Airport.
Police would not confirm Russian reports that the attacker had used a sub-machine gun.