LONDON — A Russian government official says it’s impossible to determine the source of a nerve agent used to poison a Russian ex-spy in Britain.

Georgy Kalamanov, Russia’s deputy minister of industry and trade, told Interfax news agency Thursday it’s impossible to pinpoint the agent’s origin and reaffirmed Moscow’s demand for a probe that would involve Russia.

A report by investigators at the Organization for the Prohibition of Chemical Weapons confirmed British findings that former double agent Sergei Skripal and his daughter were poisoned with a military-grade nerve agent on March 4, but didn’t identify its source.

Britain has blamed Russia for the March 4 poisoning of the Skripals with a nerve agent developed by the Soviet Union. Russia denies any involvement, saying Britain hasn’t provided evidence to support its assertion.

U.K. Foreign Secretary Boris Johnson says that only the Russians had the “means motive and record” to launch a nerve agent attack.

Johnson says “there can be no doubt what was used and there remains no alternative explanation about who was responsible — only Russia has the means the motive and record.”

A report Thursday says Organization for the Prohibition of Chemical Weapons investigators “confirm the findings of the United Kingdom relating to the identity of the toxic chemical that was used in Salisbury.”

It says the chemical was “of high purity.” The summary does not name Novichok — the name that was previously given by British Prime Minister Theresa May — but says the details of the toxin are in the full classified report.

It does not identify the source.

Britain has blamed Russia for the March 4 poisoning of Sergei Skripal and his daughter, Yulia, with a nerve agent developed by the Soviet Union. Russia denies any involvement, saying Britain hasn’t provided any evidence for its assertion.