The Russian Foreign Ministry asked the international agency that monitors chemical weapons for information Sunday about the investigation of the poisoning of a former Russian spy and his daughter in England.

A list of questions submitted to the Organization for the Prohibition of Chemical Weapons includes what sort of assistance Britain requested from the watchdog agency and which sampling procedures were used to collect the substance that sickened Sergei Skripal and his daughter, Yulia.

OPCW representatives were among a group of experts Britain asked to analyze the chemical agent involved in the poisonings. Britain claims it was the Soviet-manufactured nerve agent Novichok and has said Russia is likely responsible, which Moscow adamantly denies.

The Foreign Ministry’s request came on the same day that Russian diplomats and their families returned to Moscow on two planes after being expelled from the United States, part of the international fallout from the March 4 attack on the Skripals.

Following a wave of similar expulsions ordered by Britain and numerous allies, the United States ordered 60 Russian diplomats out of the country.

Russian news agencies said the diplomats kicked out of the United States returned on two flights that landed Sunday at Moscow’s Vnukovo Airport.

One carried diplomats from the Russian Embassy in Washington; aboard the other were diplomats from the Russian Consulate in New York and Russia’s United Nations mission.

More than two dozen countries and NATO have expelled Russian diplomats in support of Britain. Russia has ordered an equal number of most of those countries’ diplomats to leave and for Britain to reduce the staff at its Moscow embassy to the same number that Russia maintains in London.

Russia consistently has complained that Britain has not provided evidence to back up its claim of Russian involvement or that the poison that afflicted the Skripals was a Russia-developed nerve agent.

The Russian Foreign Ministry also submitted questions to British and French authorities on Saturday. The ministry did not say what actions Russia might take if the parties do not answer the questions or provide partial responses.