Four billionaires and two executives at state-owned gas and oil giant Gazprom have died since Russian troops began preparing to invade their neighbour in late January.

They include Mikhail Watford, a Ukraine-born gas and property tycoon who told friends he feared Putin’s hit list ‘for years.

Three days beforehand, Joe Biden told Volodymyr Zelensky to ‘prepare for impact’.

Less than a month after Shulman’s death, Gazprom Deputy Director, Alexander Tyulakov was found hanged at the same St Petersburg housing complex.

Three days later Mikhail Watford was found dead – and three weeks after that, medical supplies tycoon, Vasily Melnikov, was killed in the alleged murder-suicide of his wife and children.

The billionaire owner of MedCom, 43, is thought to have murdered his wife, 41, and two children aged ten and four before taking his own life.

Local investigators said there were ‘no signs of unauthorized entry into the apartment.

‘We are considering several versions of what happened, police in a Western city, Nizhny Novgorod added.

On April 18, Gazprombank Vice-President, Vladislav Avayev was found dead with his wife and daughter in their Moscow apartment.

Russian reports said the gas executive shot and killed his family before turning the gun on himself. He was reported to have tortured his wife for hours.

But Avayev’s ex-colleague Igor Volobuev said the suicide is ‘hard to believe’ and alleged it was staged.

Mr Volobuev denied that Avayev – who may have had FSB links and was found with an FSB gun after his death – had left his role as the senior Vice-President at Gazprombank, as had been widely reported.

Mr Avayev was still at the bank and would have had access to the accounts of its most elite clients, including Putin’s circle and possibly the president himself, his co-worker added.

Mr Volobuev told CNN: ‘Did he kill himself? I don’t think so. I think he knew something and that he posed some sort of risk.’

The next day, billionaire gas Executive, Sergey Protosenya was found dead in his Spanish holiday home, with his wife and daughter ‘hacked to death with an axe’.

Spanish authorities suggested that Mr Protosenya, 55, executed the pair before killing himself in an uncharacteristic fit of rage while the family enjoyed an Easter break on the Costa Brava last week.

But Protosenya’s son Fedor, 22, said his father ‘could never harm’ his family in that way.

He told MailOnline: ‘He loved my mother and especially Maria my sister. She was his princess.

‘He could never do anything to harm them. I don’t know what happened that night but I know that my dad did not hurt them.’

Mr Protosenya did not leave a suicide note and no fingerprints were found on the weapons – an axe and a knife – used to kill. There were no bloodstains on his body.

Fedor, a 22-year-old student, said the police had told him not to discuss the case.

Protosenya’s friend, Anatoly Timoshenko also told MailOnline: ‘Sergey did not do it. Sergey did not kill his family. It is impossible. I do not want to discuss what may have happened at the house that night but I know that Sergey is not a killer.’

Another friend, Roman Yuravich, added: ‘Sergey did not kill his family. I have known him for ten years. He was a happy man.

‘He loved his family. He did not kill his wife and child. I am sure.’