LONDON -- British police said Monday that they don't know what made a man and woman sick at a restaurant near where former spy Sergei Skripal and his daughter were poisoned -- but it wasn't the nerve agent Novichok.

Police officers and paramedics descended on the Prezzo restaurant in Salisbury after a man and a woman became sick on Sunday evening. Roads were cordoned off as medics in protective suits investigated.

The major response followed the March poisoning of Skripal, an ex-Russian spy, and his daughter Yulia, who were left critically ill. Two local people who came into contact with Novichok later also were poisoned, and one of them died.

Wiltshire Police said the 42-year-old man and 30-year-old woman, whose names were not released, remained in hospital under observation.

The police force said that because of the previous incidents "tests were immediately carried out which enabled us to rule out that the pair had been exposed to Novichok."

Detective Sgt. Jonathan Davies-Bateman said "tests are ongoing to understand what, if anything, the pair have come into contact with which may have caused them to fall unwell."

"At this stage, it is unclear as to whether or not a crime has been committed," he said.

Amanda Worne, who was at the restaurant, said a police officer told her the couple who fell ill were Russian. The police force declined to comment on their nationality.

British officials blame the Russian government for the attack with a banned chemical weapon on the streets of an English city. The U.K. has charged two suspects in absentia, and said they worked for Russian military intelligence.

The two men went on Russian TV last week and said they were merely tourists who had visited Salisbury to see its famous cathedral.

Health authorities say there is little risk to the public in Salisbury, but acknowledge they can't be sure whether all of the nerve agent has been found and removed.

Salisbury City Council leader Matthew Dean tweeted that there had been "a number of false alarms since the Skripal poisoning."

He said it was correct for emergency service personnel to start with a "highly precautionary approach until they know otherwise."