LONDON -- British police said Monday they believe the latest victims of poisoning by a nerve agent must have handled the material's container and been subjected to a "high dose" of the lethal poison. Metropolitan Police Assistant Commissioner Neil Basu said the death of 44-year-old Dawn Sturgess on Sunday shows that she and partner Charlie Rowley, 45, were exposed to a large quantity of Novichok, a military-grade nerve agent produced in the Soviet Union during the Cold War.

Basu said the working theory is that their exposure was linked to the earlier Novichok attack in March on ex-Russian spy Sergei Skripal and his daughter Yulia, who have both survived despite extended hospitalizations.

Britain has blamed Russia for the poisonings, but Moscow has strongly denied any involvement. The wide investigation is now a homicide inquiry. Basu expressed sorrow for the death of Sturgess, who had three children.

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She leaves behind two sons, ages 19 and 23, and an 11-year-old daughter, BBC News reports.

"Her death has only served to strengthen the resolve of the investigations team," he said, saying the immediate police priority is to find any container that may be the source of the Novichok.

He said no one else in the Amesbury and Salisbury region where the couple lived in southwestern England has shown any sign of Novichok poisoning.

Forensic investigators, wearing protective suits, emerge from the rear of John Baker House, after it was confirmed that two people had been poisoned with the nerve-agent Novichok, in Amesbury on July 6, 2018. Henry Nicholls / REUTERS

More than 100 police are working to try and search all areas where Sturgess and Rowley had been before they became ill nine days ago. The search is focused on their homes and a park in Salisbury. Rowley remains hospitalized in critical condition.

Britain blames the Russian state for the attack on Skripal and his 33-year-old daughter -- an allegation Moscow has repeatedly denied.

A Kremlin spokesman Monday expressed condolences over Sturgess' death.

Dmitry Peskov told reporters that Moscow "is deeply concerned" over the poisoning cases. Peskov said the attacks present a danger not only inside the U.K., but also in Europe as a whole, and said linking Russia to the poisoning would be "absurd."

Asked whether the death could cloud the upcoming U.S.-Russia summit in Helsinki next week, Peskov replied that the poisoning "has no relation" to the meeting. He said, "It's Britain's problem and the problem of how interested Britain is in a real investigation."

Moscow says London has declined its offers for a joint investigation into the poisonings.