LONDON — A British police official 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.

Personnel in hazmat suits work at the scene of a nerve agent attack on a former Russian spy and his daughter in Salisbury, England, on Sunday. Andrew Matthews/PA via AP

Health officials continue to 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 government is found to be responsible.

“Multiple people have been treated, around 21 people, including the man and the woman found on the bench,” Wiltshire acting police chief Kier Pritchard told Sky News, referring to Skripal and his daughter, who were found unconscious.

Pritchard said that of the new total, “a number” of them got hospital treatment including blood tests, support and advice.

Previously, British authorities had said only that “several” people had sought treatment.

Police haven’t provided details on the nerve agent that was used, and the ex-spy and his daughter remain in critical condition.

British Home Secretary Amber Rudd said whoever is behind the attack is guilty of a “brazen and reckless act.”

She said Britain would respond strongly when it is clear who’s to blame.

She said that enormous resources were being used to determine who poisoned Skripal, 66, and his 33-year-old daughter, Yulia.

They were found unconscious on a bench, triggering a police inquiry headed by counterterrorism detectives.

A police officer who came to their aid is hospitalized in serious condition, though he is conscious and talking, Rudd said. He was identified Thursday as Sgt. Nick Bailey.

“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.”

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 killing of another former Russian spy, who was poisoned in London with radioactive polonium-210 in 2006.

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