British authorities investigating the poisoning of two people have found a bottle believed to have contained Novichok, a nerve agent that was also used against a former Russian spy and his daughter, the Metropolitan Police said Friday.

Dawn Sturgess, a 44-year-old woman who was exposed to the poison, died in a hospital on Sunday. Charlie Rowley, her 45-year-old partner, was said to be in critical condition before he regained consciousness. He is in serious condition but is stable, police said Friday.

The small bottle was found during searches of Rowley's house in Amesbury on Wednesday, police said in a statement. It was tested, and scientists confirmed the substance inside was Novichok, a military-grade, Soviet-produced nerve agent that was used in an attack on the ex-spy, Sergei Skripal, and his daughter, Yulia. Sturgess and Rowley were hospitalized in late June, authorities have said.

Authorities said they were investigating where the bottle came from, how it turned up in Rowley's house, and whether the substance came from the same batch that poisoned the Skripals, who survived the March attack. They have been released from a hospital, but are in a secret protected location, according to the Associated Press.

"This is clearly a significant and positive development," Metropolitan Police Assistant Commissioner Neil Basu said in a statement. "However, we cannot guarantee that there isn't any more of the substance left and cordons will remain in place for some considerable time."

Investigators have spoken with Rowley and will continue to talk with him to "further establish how he and Dawn came to be contaminated," police said. In a previous statement, police said there was no evidence that Sturgess or Rowley visited any of the sites that were decontaminated after the poisoning of the Skripals.

The Skripals were found unconscious on a park bench on March 4 in Salisbury, around 7 miles from Amesbury.

British Prime Minister Theresa May has blamed the Russian government for the attack on the Skripals. The Kremlin has repeatedly denied any involvement.