LONDON — A critically ill former Russian spy and his daughter were victims of a nerve agent, British police said, increasing the probability that their attack was the work of another state.

Sergei Skripal and his daughter Yulia were found unconscious on a bench Sunday in Salisbury, southwest of London. A police officer who was on the scene early is also in hospital in a serious condition.

The pair had been “targeted specifically” and the case is being treated as attempted murder, Metropolitan Police Assistant Commissioner Mark Rowley told reporters in London Wednesday. The police said in an emailed statement that “government experts have identified the specific nerve agent used which will help identify the source.”

Skripal was convicted in 2006 of passing the identities of Russian agents in Europe to the U.K.’s Secret Intelligence Service, MI6, according to a person familiar with the case. Russian authorities said payments totaling $100,000 were made into a Spanish bank account in return for his work for the U.K. He was sentenced to 13 years in jail, but in 2010 was pardoned and sent to Britain, in a swap deal involving agents who had been arrested in the U.S.

Foreign Secretary Boris Johnson told Parliament on Tuesday that Russia had become a “malign and disruptive force,” and that Britain would be likely to increase sanctions if evidence emerged that Russia was behind the attack.

The incident has uncomfortable echoes of the case of Alexander Litvinenko, a Russian ex-spy who was murdered in 2006 after his tea was spiked with radioactive polonium. In 2016, a judge ruled that Russian President Vladimir Putin probably approved the murder. Russia dismissed the U.K. inquiry at the time as a “politicized farce.”