21 SEEK MEDICAL TREATMENT AFTER EX-SPY POISONED IN U.K.,

LONDON — About 21 people have received medical treatment after a nerve-agent attack on a former Russian spy, British police said Thursday, as the U.K. vowed strong action against whoever was responsible for the “brazen and reckless” act.

Three people remain hospitalized after the poisoning Sunday in the southern English city of Salisbury — former spy Sergei Skripal, his daughter and a British police officer who tried to help them.

Health authorities said there is little risk to the wider public. But Wiltshire county acting police chief Kier Pritchard said “around 21 people” have had treatment, including the Skripals.

Pritchard said “a number” of the 21 were having “blood tests, support and advice.” Previously, authorities had said only that “several” people had sought treatment.

The ex-spy and his daughter remain in critical condition in a Salisbury hospital. A police officer who came to their aid, Sgt. Nick Bailey, is hospitalized in a serious condition, but conscious and talking, officials said.

British Home Secretary Amber Rudd said enormous effort is being devoted to determining who poisoned Skripal, 66, and his daughter, Yulia, 33. They were found unconscious on a bench, triggering a police inquiry headed by counterterrorism detectives.

“The use of a nerve agent on British soil is a brazen and reckless act,” Rudd told Parliament. “This was attempted murder in the most cruel and public way.”

Rudd said Britain would respond strongly when it is clear who is to blame.

The Russian Embassy in London, which has mocked other British politicians for suggesting Russian involvement, tweeted that it agreed with Rudd: “First evidence then conclusions on Mr. Skripal’s case. Responsible political approach.”

Police have refused to publicly speculate on who is behind the attack, but many experts have focused on Russia because of the similarity to the 2006 killing of another former Russian spy who was poisoned in London with radioactive polonium-210.

A public inquiry found that Russia was responsible for killing Alexander Litvinenko, and that President Vladimir Putin probably approved it. The Russian government has denied any involvement in either case.