London(CNN) Investigators are looking into whether the poison used on former Russian double agent Sergei Skripal and his daughter Yulia could have been left in multiple receptacles in and around the English town of Salisbury, sources say.
CNN reported last week that British police have identified two suspects believed to have left the UK on a commercial flight shortly after the attack.
Now, authorities are looking into whether there was a drop team that planted the Novichok and a second team that carried out the attack -- and whether that drop team left multiple sources of Novichok in the vicinity, sources say.
The new details emerged as police on Wednesday admitted there could be more Novichok left in the Salisbury area.
"What we can't tell, and probably will never be able to tell, is actually is there anything else out there," said Deputy Chief Constable Paul Mills, of Wiltshire Police, during a press briefing.
"So, all we can do is be intelligence-led, go to the locations where we know they've actually been and be meticulous with the searching," he added.
Novichok victim Charlie Rowley told CNN affiliate ITV news on Tuesday that he had found a box wrapped in cellophane that he had opened, and that he and his partner Dawn Sturgess were both exposed to the substance inside the bottle contained in the box before falling ill. Sturgess died several days later.
British public health authorities have consistently urged the public to be wary of touching any foreign objects. "To be clear: do not pick up anything that you haven't dropped yourself," Public Health England says on their website.
The Home Office, the Foreign Office and the Department for Environment, Food & Rural Affairs -- which is handling the cleanup operation -- all referred CNN to Metropolitan Police, which is leading the investigation.
The Metropolitan Police told CNN the case was very much an active investigation and that it was not prepared to discuss specific lines of inquiry.
Novichok is a rare Soviet-era military-grade nerve agent. The British government blames the Skripals' poisoning on Russia.
The suspects were identified by UK police, who have been combing through months of surveillance camera footage from UK airports and from Salisbury, the town where the Skripals were poisoned on March 4.
Using facial recognition technology, authorities discovered two "fresh identities" -- individuals not known to have been spies or used in other attacks -- that source added.
Investigators cross-checked that information with the manifest of a flight on which the suspects were believed to have left Britain. They are thought to have traveled under aliases, the source said. It is not clear whether the pair are Russian.
An Aeroflot plane at Heathrow Airport in London was searched on March 30, an action the British government described at the time as "routine."
The attack on the Skripals led to a spiraling diplomatic dispute with Russia. The UK expelled 23 Russian diplomats, and more than 20 other countries, including the United States, followed suit.
In a letter to NATO secretary general Jens Stoltenberg in April, UK national security adviser Mark Sedwill said that Russia was "highly likely" responsible for the attack and that there was "no plausible alternative explanation."
Russia has repeatedly denied any involvement in the attack and has put forward multiple theories as to who may have been behind it.
The Skripals were found unconscious on a bench in the city of Salisbury on March 4. They were treated for exposure to the military-grade nerve agent and discharged separately from hospital several weeks later. Police believe they came into contact with the toxin via the door handle of Sergei Skripal's house.
In April, the Organisation for the Prohibition of Chemical Weapons confirmed the UK's finding that Novichok was used in the attack. Last week, the watchdog was asked by the UK to deploy another team to determine the nature of the substance to which Sturgess and Rowley were exposed.
Sturgess, 44, and her partner Rowley, 45, both residents of Amesbury, a town eight miles north of Salisbury, were hospitalized on June 30. Rowley was discharged last week.
Police said the couple had been exposed to Novichok after touching a contaminated item, but said there was no suggestion they had been deliberately targeted.
The Skripal case has drawn comparisons to the poisoning of former Russian spy Alexander Litvinenko, who died a slow death after drinking tea laced with highly radioactive polonium-210 in a Mayfair hotel of London in 2006.
A detailed UK inquiry later concluded that Russian President Vladimir Putin probably approved the operation by Russian agents to kill Litvinenko. The Russian Foreign Ministry dismissed the UK investigation as politically motivated.