A Soviet general behind the development of a deadly nerve agent suspected of being used in a poisoning attack last week in the UK had raised concerns in Israel in the 1990s. The Jewish state was worried that he was trying to sell his knowledge to Syria, and he later died in mysterious circumstances, according to a report published Friday.

The general, Anatoly Kuntsevich, described as a leading chemical weapons expert, had led the development of a highly potent Soviet-designed nerve agent called Novichok, which Britain says was used on former double agent Sergei Skripal and his daughter on March 4.

Amid the collapse of the Soviet Union, Kuntsevich began trying to sell his knowledge to the Syrians, according a report in the Ynet news site by Israeli journalist and author Ronen Bergman, whose “Rise and Kill First: The Secret History of Israel’s Targeted Assassinations,” was published earlier this year.

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“It would seem that his business with the Syrians was not a government initiative but rather an attempt by him to look after his own interests,” Bergman wrote, saying that Kuntsevich received “huge sums of money.”

Soviet soldiers working on a Sarin bomb. Shikhany CW facility, USSR, 1987. First pic: MOD. Others © Hans de Vreij pic.twitter.com/C8rrxIspFW — Hans de Vreij (@hdevreij) April 10, 2017

Israel repeatedly warned Moscow, but to no avail. “It was believed that [Russian president Boris] Yeltsin either could not, or did not want, to intervene,” the report said.

Bergman cites the book “The Volunteer,” which was published in Canada by Michael Ross, in which he claimed to be a Mossad agent and said he was repeatedly dispatched to warn senior Russian officials about Kuntsevich’s activities. Again, without any results.

“Israel was furious. On 29 April, 2002, in circumstances that remain unknown, Kuntsevich died during a flight from Aleppo to Moscow,” Bergman wrote. “The Syrians appear to be confident that the Israeli intelligence had succeeded in reaching and poisoning the general.”

Syria agreed to give up its chemical arsenal in 2013 when then US president Barack Obama threatened missile strikes in retaliation for a chemical attack on a rebel-held suburb of Damascus during the country’s civil war. The attack is believed to have killed more than 1,000 people. Obama abandoned talk of attacking Syria after President Bashar Assad agreed to the weapons surrender.

However, Syria has since repeatedly been accused of using chlorine gas in attacks.

Last week’s attack with the nerve agent has caused a major rift between Russia and the UK.

Skripal, once a Russian double agent, along with his daughter Yulia, and a British police officer, were poisoned with a rare and powerful nerve agent. Skripal and his daughter remained in critical condition as of Thursday, and the police officer was considered seriously ill.

Russia insists it had no motive to target Skripal with what Britain says was the first such attack in Europe since World War II.

Skripal had taken his daughter, who was on a visit from Moscow, out for lunch before they both collapsed on a bench.

Many Russians remain skeptical that the state was responsible and some analysts have not ruled out the involvement of ordinary criminals or rogue agents.

On Thursday, deputy foreign minister Sergei Ryabko denied that Russia even had a program to develop the Novichok nerve agent.

“I want to state with all possible certainty that the Soviet Union or Russia had no programs to develop a toxic agent called Novichok,” he told Interfax news agency.

He slammed people “distributing information that the program allegedly existed,” an apparent reference to Soviet chemist Vil Mirzayanov, who first revealed the existence of that class of ultra-powerful nerve agents.

Mirzayanov, who now lives in the United States, says Moscow invented the highly toxic nerve agent during the Cold War and used to produce it in a Moscow-based institute where he worked until the early 1990s.

“We ended all research in the sphere of new military toxic agents after joining the [Chemical Weapons] Convention, and last year… all stockpiles of toxic agents were destroyed,” said Ryabkov.

He said the United States has failed to do the same.

“I hope that debates around the tragedy in Salisbury will not be a new pretext for the US to depart from what they have to do within the framework of their own obligations,” he added.

In a rare joint statement, US President Donald Trump, French President Emmanuel Macron, German Chancellor Angela Merkel, and British Prime Minister Theresa May said “there is no plausible alternative explanation” to Russian responsibility.

The leaders said the use of a chemical weapon is “an assault on UK sovereignty” and “a breach of international law.”

On Wednesday, May expelled 23 Russian diplomats from the UK, severed high-level contacts with Moscow, and vowed both open and covert actions following the attack, plunging UK-Russia relations to a level not seen since the Cold War.

Russian Foreign Minister Sergey Lavrov said Thursday that Moscow would “certainly” expel some British diplomats in a tit-for-tat response.