Son of Russian oligarch found dead with wife and daughter doesn't believe the police's murder-suicide theory

Police suspect a murder-suicide after a Russian oligarch was found dead alongside his wife and daughter.

But the oligarch's surviving son told MailOnline that his father is "not a killer."

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Murder-suicide is the prevailing police theory in Sergey Protosenya's case.

The son of a Russian oligarch who was found dead alongside his wife and child has said his father is not a killer, pouring cold water on the primary theory of the police investigating the case.

Sergey Protosenya, 55, was found hanged in a villa in Lloret de Mar in Spain's Catalonia region on April 12. The bodies of his wife, Natalya, and teenage daughter, Maria, were also found with stab wounds nearby.

Protosenya, who had been retired at the time of his death, formerly served on the board of Novatek, a major Russian gas company. He was worth an estimated $440 million, Spanish newspaper La Vanguardia reported.

Insider confirmed via a local official that the primary working theory of the Mossos D'Esquadra, the Catalonian police force investigating the deaths, is that of murder-suicide committed by Protosenya.

But his surviving son Fedor told MailOnline that his father "is not a killer."

Protosenya's body was found with a bloodied knife and an ax nearby, but there were no bloodstains on his person and no suicide note, the Spanish outlet El Punta Vui reported.

Fedor was in the family's home in France at the time of the deaths, and alerted police to the villa after they did not answer their phones, Spanish TV outlet Telecinco reported. They were in Lloret de Mar for an Easter break, the outlet reported.

Fedor said that Protosenya loved his mother "and especially Maria my sister," MailOnline reported.

"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," Fedor said.

Fedor's view was underscored on April 21 by Protosenya's former employer Novatek, which issued a statement calling Protosenya "an outstanding person and a wonderful family man." The statement also suggested that media reporting around the murder-suicide theory was solely speculative.

Josep Milan, police chief for the Catalonian region, told MailOnline that the evidence suggested this was a murder-suicide, with Protosenya as the perpetrator.

"The investigators are focusing on a person who has committed suicide and two people who were killed at home, this is what we are looking at," he told the outlet.

Protosenya's death came just a day after police found the bodies of another Russian oligarch, Vladislav Avayev, as well as his wife and child in their Moscow apartment, state-run news agency Tass reported. There is no evidence of a connection between the cases.

All three died of gunshot wounds, Tass reported, and cited an anonymous law-enforcement source as saying that Avayev killed his wife and daughter before killing himself.

Avayev was the former vice president of Gazprombank, the Russian bank set up for the state-run gas company Gazprom.

Igor Volobuev, a former vice president at Gazprombank, told Russia's The Insider news outlet that he did not know Avayev personally but said he did not believe Avayev killed himself.

"I never knew him and never heard of him before [the deaths]," Volobuev said. "I do not believe that he could have killed his wife and daughter. I think it's staged. Why? It is hard to say. Maybe he knew something and represented some kind of danger."