LONDON — The international chemical weapons watchdog says it is returning to Britain collect more evidence in the poisoning probe of a couple sickened by the nerve agent Novichok.

The Organization for the Prohibition of Chemical Weapons says a technical assistance team will collect additional samples and send them to designated laboratories.

The organization was asked to investigate after two people fell ill in June in the town of Amesbury in southwestern England.

Dawn Sturgess and her partner, Charlie Rowley, were sickened months after former Russian spy Sergei Skripal and his daughter were poisoned with Novichok in the nearby city of Salisbury. Sturgess later died.

The move comes amid reports in the Guardian newspaper that authorities are preparing to submit an extradition request to Moscow for two Russians suspected of carrying out the Salisbury attack.