A former Russian spy in the UK and his visiting daughter fell critically ill on Monday, suffering symptoms of poisoning. Several police officers who arrived at the scene were also hospitalized and assessed. The pair remain in critical condition as UK police continue in efforts to determine what lay behind the mysterious incident.

HQ of UK intelligence service MI5

HQ of UK intelligence service MI5 Photo Credit: Cnbrb/Wikipedia

A man and a woman found unconscious on a bench in England on Monday were identified as a former Russian spy who was extradited to the UK and his daughter. The pair had fallen critically ill after reportedly suffering symptoms of poisoning.

Sergei Skripal, 66, and daughter Yulia Skripal, 33, were found near a shopping center in Salisbury and were taken to a nearby hospital. Yulia lives in Russia but came to visit her father in the UK, according to the BBC. Her relatives had reportedly not heard from her for two days.

The case is being investigated by local authorities as they continue in efforts to establish what substance may have caused the pair to collapse. “Clearly it’s a very unusual case and the critical thing is to get to the bottom of what has caused this incident as quickly as possible,” said Metropolitan police assistant commissioner Mark Rowley in a statement on Tuesday. “As you would expect, the specialist resources that sit within the counter-terrorism network that I coordinate across the country and other partners are working with Wiltshire police to get to the bottom of that as quickly as possible.”

Skripal arrived in the UK in 2010 along with his wife as part of a “spy exchange” deal with Russia. His wife reportedly died two years later due to cancer. According to the BBC, Skripal’s son also died recently on a visit to St Petersburg with his girlfriend, aged 43.

Dmitry Peskov, spokesman of Russian President Vladimir Putin told reports that Moscow did not have any information on what could have led to the incident, which he described as “tragic.” The Wiltshire Police confirmed that several emergency services personnel were also medically assessed immediately after the incident. All but one have been released from the hospital, according to the statement.