There could be revenge attacks against Oniani's group, as well as score-settling between members of Usoyan's own grouping as they jockey for influence following their Don's death.
There is also a somewhat fanciful theory linking the killing to the triple assassination of Kurdish separatists in Paris last week. Usoyan, an ethnic Kurd, was rumoured to be one of the suppliers of illicit weapons to Kurdish separatist movements fighting in Turkey.
According to Mark Galeotti, a professor at New York University who specialises in the Russian underworld, Usoyan's nephew Miron has been primed to take over from him since a 2010 failed assassination attempt left Usoyan fearful for his life.
"But a lot of people aren't sure if he is up to it," Galeotti said.
"Usoyan built his criminal network on very personal links and it is not clear whether these loyalties will simply transfer."
Unlike the more sophisticated organised corruption networks that have sprung up in the past decade and are closely interwoven with government structures, underworld figures like Usoyan are a dying breed.
"Usoyan was very much a Don Corleone figure," says Galeotti.
"He had enough links with political structures to ensure that he had protection but he didn't actually work directly with politicians."
Moscow is still hit by occasional contract killings, such as the 2009 murder of Vyacheslav Ivankov, a mafia boss known as the Little Japanese, and the failed 2010 attack in which Usoyan was injured. Such attacks have become rarer than they were in the 1990s but this latest attack could spark revenge killings.
"The killing of such an influential figure cannot go without revenge from his supporters," said Alexander Khinshtein, a Russian MP, yesterday.
"This is likely to bring about a new wave of attacks and murders."
In news reports on Russian television, Usoyan was openly referred to as a "thief-in-law" and a "criminal authority". He was one of the last of the major "thieves-in-law", the Soviet-era crime bosses whose culture and ethics originated in the Gulags, and his lifestyle was different to the newer class of Russian rich.
He was an old-school mafia boss who ate at the same kitschy Azerbaijani restaurant most days, where he would receive visitors and other clan bosses keen to settle disputes, and where he was assassinated. He rarely travelled abroad and although he travelled in a four-car motorcade for security he was not ostentatious with his ill-gotten wealth.
He was hit in the neck by a sniper hiding in a building across the street from the restaurant, where he had just had lunch. Police said the gunman used a Val, a sniper rifle with a silencer that has a range of up to 400m and is used by the Russian special forces and elite units of the army.
The sniper appears to have had at least one accomplice and to have shot from a fifth-floor window in the stairwell of an apartment block across the street. Police believe they have captured the killer on camera entering the building with a large bag, but the man is wearing a winter coat and hat and his features aren't visible.
The "control shots" meant to ensure that the mafia boss was dead instead hit a waitress from the restaurant, who is in a critical condition in hospital after losing four litres of blood. But one bullet was enough to finish off Usoyan, who died after being rushed to hospital by his bodyguards in his own motorcade.
No details about Usoyan's funeral have been released yet. It is likely to require a huge police presence as criminal bosses from across Russia come to pay their respects.
Kingpins in crime
Aslan Usoyan: Born in 1937 in Tbilisi, Georgia, Usoyan was an ethnic Kurd who rose to become one of post-Soviet Russia's most powerful crime bosses. In the only interview he has ever given, he described himself as a pensioner, but did not deny that he was a "thief-in-law".
Semyon Mogilevich: In a class of his own, Mogilevich is believed to be the most powerful underworld financier and money launderer with wide connections across the crime and political worlds in Russia and neighbouring countries. He is on the FBI's 10 Most Wanted list, but resides unmolested in Moscow.
Tariel Oniani: Known as Tarot, Oniani hails like many of the Soviet-era crime leaders from the republic of Georgia. It is believed that his gang was involved in a bloody turf war with Usoyan's gang, primarily over lucrative construction contracts involved in the 2014 Winter Olympics in Sochi. He was jailed for 10 years on kidnapping and extortion charges in 2010.
Vyacheslav Ivankov: Also born in Georgia, Ivankov was an ethnic Russian but because of his Asian features was named "the Little Japanese". Shot dead in Moscow in 2009, apparently after mediating a dispute between Oniani and Usoyan.
- Independent
