LONDON — The British police announced on Friday that they had found “a small bottle” containing a Novichok nerve agent, providing a possible break in the four-month-old investigation into an attack on a former Russian spy and the inquiry into the poisoning of two British citizens.
The bottle was found in the home of Charlie Rowley, 45, in Amesbury, England. He and his partner, Dawn Sturgess, 44, were exposed to the nerve agent on July 1.
“This is clearly a significant and positive development,” Assistant Commissioner Neil Basu, the head of the national counterterrorism squad, said of the discovery on Friday. “However, we cannot guarantee that there isn’t any more of the substance left and cordons will remain in place for some considerable time.”
He said the police could not reveal any more details about the bottle, which was being examined by experts at Porton Down, the British government’s laboratory for chemical and biological weapons.
Ms. Sturgess died on Sunday evening, and officials have begun a murder inquiry. Mr. Rowley has regained consciousness, and he is speaking to investigators.
The death was the first fatality linked in Britain to the release of a Soviet-developed Novichok nerve agent in the southern city of Salisbury, site of the attack on March 4 on the former spy, Sergei V. Skripal, and his daughter, Yulia Skripal.
The bottle could provide valuable information to investigators, if it was created in a military laboratory.
The British police say they believe that Mr. Rowley and Ms. Sturgess touched a container left behind by a team of would-be assassins sent to attack Mr. Skripal.
The emergence of additional victims so long after the first poisoning alarmed residents of the area, and the authorities quickly cordoned off five sites that they feared might have been contaminated.