(CNN) Authorities in the UK have opened a murder investigation after a British woman died from exposure to a Soviet-era nerve agent that had previously sickened a former Russian spy and his daughter.

Prime Minister Theresa May said that authorities were "working urgently to establish the facts of this incident," adding that she was "shocked and appalled" by the death of 44-year-old Dawn Sturgess on Sunday evening.

Speaking at Scotland Yard on Monday, Neil Basu, the assistant commissioner for counterterrorism with London's Metropolitan Police, said there was no evidence that the nerve agent that caused Sturgess' death was linked to the attack on the Sergei and Yulia Skripal, but that this remained the "main line of enquiry."

UK Home Secretary Sajid Javid last week called on Russia to provide an explanation for the incident after it was confirmed that Sturgess and another person, 45-year-old Charles Rowley, were exposed to Novichok , the same substance that sickened the Skripals in March.

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

Russia denies any involvement in the Skripals' case. Russia's United Nations Ambassador Vassily Nebenzia blasted the UK during a UN Security Council meeting in April , calling the allegations a "fake story."

Over 20 countries, including the US, expelled Russian diplomats in a show of support for the UK.

Despite dismissing concerns over Russian President Vladimir Putin at a rally in Montana last week, US President Donald Trump said in March that he believed the UK government's theory that Russia was probably responsible for the attack on the Skripals.

"It sounds to me like it would be Russia, based on all the evidence they have," Trump told reporters on March 13. The White House later said it was expelling 60 Russian diplomats as retaliation for Moscow's alleged actions.

Prime Minister May is hosting Trump later this week, just days before the US President is scheduled to meet Putin in Finland.

Kremlin spokesman Dmitry Peskov dismissed any Russian involvement in the Amesbury poisoning, calling it "more of a UK problem."

Asked by CNN to comment on remarks by a Russian official who suggested that the poisoning had been "done on purpose" to derail the World Cup and ongoing summit preparations," Peskov said it was "absurd" to mention Russia in connection with the incident, but added, "the nature of the incident raises concerns and poses a big threat."

Charles Rowley and Dawn Sturgess are seen in images taken from their Facebook accounts

Contaminated item not yet located

Police believe Sturgess and Crowley were exposed to the nerve agent Novichok after handling a contaminated item. Both were admitted to Salisbury District Hospital nine days ago after falling ill, the same hospital where the Skripals were treated.

"Our focus and priority at this time is to identify and locate any container that we believe may be the source of the contamination," said assistant commissioner Basu Monday.

"Their reaction was so severe, it resulted in Dawn's death and Charlie being critically ill. This means that they must have got a high dose and our hypothesis is that they must have handled a container we are now seeking."

There is no evidence that either Sturgess or Rowley visited any of the sites that were decontaminated following the attack in March, according to authorities, who say they are continuing to build a time line of the couple's movements.

Police initially cordoned off six sites in Amesbury and Salisbury and later expanded their probe of one of those sites -- low-income housing in the center of Salisbury -- to include part of the nearby street and inside the building.

Dawn Sturgess is understood to have links to John Baker House, which CNN saw residents leaving under instruction from police. The social housing is located about half a mile from a restaurant where the Skripals dined before falling ill.

Very difficult to treat

Sturgess initially collapsed at an address in Amesbury on June 30 and was taken to the hospital, police said. Five hours after her collapse, police said, an ambulance was called back to the same address for Rowley, who also fell ill and was taken to hospital.

Salisbury District Hospital's medical director, Dr. Christine Blanshard, said Sunday that the staff "worked tirelessly to save Dawn."

"Dawn's death illustrates just how difficult it is for the doctors treating her," said Alastair Hay, professor of environmental toxicology at the University of Leeds. "Even though they are now the most experienced team in the world for dealing with this nerve agent, there are still limits to how much they are able to do."

"This latest horrendous turn of events has only served to strengthen the resolve of our investigation team as we work to identify those responsible for this outrageous, reckless and barbaric act," said the Metropolitan Police's Basu on Monday.

Basu said that, while he cannot "offer guarantees," the "risk to the general public at this time remains low."

According to Basu, 21 people have presented with health concerns, but all have been screened and given the "all clear."

Among those 21 was a police officer who sought medical advice Saturday in connection with the investigation.

A very rare nerve agent

Nerve agents kill by affecting the nervous system in various ways.

Novichok, which works by causing a slowing of the heart and restriction of the airways, is one of the world's rarest nerve agents . Very few people outside Russia have experience with it.

It was developed in secret by the Soviet Union during the Cold War in the 1980s as a means of countering US defenses against chemical weapons, but was revealed to the world by former Soviet scientist and whistleblower Vil Mirzayanov.

Mirzayanov told CNN that Novichok is up to 10 times as potent as VX, the weapon used to kill North Korean leader Kim Jong Un's half-brother, Kim Jong Nam , in 2017.