LONDON (AP) — A British police official on Thursday updated the number of people who sought treatment after a nerve agent attack on an ex-Russian spy, saying "around 21" had been given medical help and support.

Only three people remain hospitalized after the poisoning Sunday in the southern English city of Salisbury — ex-spy Sergei Skripal, his daughter and a British police officer who tried to help them. Health officials insist there has been a low risk to the public.

Authorities haven't said who launched the attack, but U.K. officials have warned of a strong response if the Russian governmen...