MOSCOW (AP) — Nearly two dozen Russian diplomats expelled by Britain over the poisoning of an ex-spy headed home Tuesday, while a scientist involved in the creation of the nerve agent said it could be manufactured by other countries.

Former Russian double agent Sergei Skripal and his daughter, Yulia, are in critical condition in the English city of Salisbury after being poisoned on March 4. Britain said they were poisoned with a class of nerve agent known as Novichok and blamed Russia for the attack.

Russian scientist Leonid Rink told the state RIA Novosti news agency that Britain and others could easily synthesize Novichok after chemical expert Vil Mirzayanov emigrated to the U.S. and revealed its formula.

"It's easily available to professionals, and there is no problem for Britain, the U.S. and other developed nations to create such weapon," he said.

Rink said Novichok had a different name when it was designed in the Soviet Union, arguing that British officials used the name Novichok to convince the public that Russia was to blame for the poisoning.

Britain has dismissed claims the nerve agent could come from the U.K. On Sunday, Russia's ambassador to the EU suggested the nerve agent could have come from Britain's chemical weapons research facility, Porton Down. The British government said that was "nonsense."

On March 14, British Prime Minister Theresa May gave the 23 diplomats — whom she said were undeclared intelligence agents — a week to leave Britain. Russia responded by expelling the same number of British diplomats, who are expected to leave in the coming days.

Diplomats and their families emerged from the Russian Embassy in west London with suitcases, bags and pet carriers. Some hugged before boarding vehicles to Stansted Airport near London for a flight to Moscow that departed Tuesday afternoon.

Russia has fiercely denied any involvement, saying that it had no motive to kill Skripal, who was convicted of spying for Britain but released in a 2010 spy swap. It said that it had completed the destruction of its chemical arsenals last year under international oversight.

White House spokeswoman Sarah Huckabee Sanders said the poisoning didn't come up in President Donald Trump's phone call Tuesday with Russian President Vladimir Putin. Sanders said the call, in which Trump congratulated Putin on his re-election Sunday, was meant to discuss areas of cooperation between the two countriees.

Putin's spokesman, Dmitry Peskov, dismissed demands by Britain and its allies that Russia prove its innocence, saying that it's Britain who must provide proof.

"Let's stay sober-minded and first of all wait for proof from Britain" that Russia was to blame, he told reporters.

Rink said Britain has refused to provide a sample of the agent it said was used in the poisoning because tests would reveal that it hadn't come from Russia. He said each lab has its own chemical "signature," allowing experts to trace its origin. "It would be immediately clear that it wasn't 'cooked' in Russia," he said.

Britain says experts from the Organization for the Prohibition of Chemical Weapons are taking samples of the nerve agent, which will be tested in international laboratories.

Britain's National Security Council was meeting Tuesday to consider possible further measures against Russia. May and other European Union leaders are due to discuss the poisoning at a summit Thursday. The EU on Monday condemned the poisoning and called on Russia to "address urgently" British questions about the Novichok nerve agent program.

The Russian Foreign Ministry accused Britain and other EU member states of developing similar nerve agents, and said Britain's government is stirring up "media hysteria" around the case to distract attention from troubles in negotiating the country's exit from the EU.

"Instead of cooperating with us ... they are simply slinging mud at us," Deputy Foreign Minister Sergei Ryabkov said. "A great country that has fallen so low only causes pity."

Ministry spokesman Maria Zakharova emphasized that Britain has failed to provide any proof of Russian involvement, limiting itself to "boorish accusations." She said the Russian Foreign Ministry is summoning ambassadors Wednesday to present Moscow's arguments and raise questions about the case.

The British military and police are continuing to search for clues around Salisbury into what happened. International chemical weapons experts are due to take samples of the nerve agent.

British police investigators say it may take months to complete the widening inquiry. The focus is on the movement of the Skripals in the hours before they were found unconscious on a bench in the city 90 miles (145 kilometers) southwest of London. A police officer who came to their assistance is in serious condition.

"This is going to be frustrating for people," said Neil Basu, head of counterterrorism at the Metropolitan Police. "It is going to take weeks, possibly months to do this."

___

Lawless reported from London. Kate de Pury and Vladimir Isachenkov in Moscow contributed.