(CNN) A small bottle discovered in the home of Novichok victim Charlie Rowley has been identified as the source of the nerve agent that killed his partner, London's Metropolitan Police said.

It is not clear where Rowley found the bottle, or whether it is the same batch of nerve agent that poisoned Russian citizens Sergei and Yulia Skripal.

Police opened a murder investigation after Rowley's partner, Dawn Sturgess, 44, died Sunday from exposure to the Soviet-era poison.

Rowley, 45, is still hospitalized. On Wednesday, the hospital said his condition had been upgraded from critical to serious.

"Charlie still has some way to go to recover, but the progress we've seen so far gives us cause for optimism," said Lorna Wilkinson, director of nursing at Salisbury District Hospital. He is conscious, officials said.

Public Health England has advised citizens not to pick up strange items made from glass, metal or plastic.

Officials say: "If you didn't drop it, then don't pick it up."

Neil Basu, the head of UK Counter Terrorism Policing, said there is no guarantee that all of the substance has been found. Search cordons remain in place.

Sturgess and Rowley fell ill in the town of Amesbury in Wiltshire, about eight miles north of Salisbury, where the Skripals were poisoned. Yulia Skripal was discharged from the hospital in April and her father was released a few weeks later.

People poisoned with Novichok and other nerve agents essentially die due to secretions -- vomiting, diarrhea and urinary incontinence occur all at once -- said Dr. Peter R. Chai of the Division of Medical Toxicology in the Department of Emergency Medicine at Brigham and Women's Hospital in Boston.

Supportive care includes oxygen; anti-seizure medication, atropine, used for some poisoning patients; and pralidoxime chloride, given to inhibit poisoning including by nerve agents.

Atropine works in two ways, Chia said. Along with drying out secretions, it increases the heart rate, which slows after exposure to a nerve agent. The antidote is a nitrogen-containing chemical compound known as an oxime.

Novichok and other nerve agents bind to a receptor site in the brain where they disrupt cholinesterase, a type of enzyme needed for proper functioning of the nervous system, Chai explained.