British intelligence is looking for a doctor known as Dr. Novichok who is said to have accompanied the hit men sent to poison former spy Sergei Skripal.
The security services are looking for the doctor along with a fourth man who is believed to have protected the Russians behind the attack of Salisbury's nerve agent.
It comes days after the identities of the suspects were given as Ruslan Borishov and Alexander Petrov, both of whom the police believe are fictitious names.
Dr. Novichok could have been involved to avoid cross-contamination, a Whitehall source told The Sunday Mirror.
The intelligence chiefs are looking for a doctor nicknamed Dr Novichok who is believed to have helped Alexander Petrov (left) and Ruslan Borishov (right)
Poison suspects have previously claimed that they were only in Salisbury to visit their cathedral (pictured)
Another role they believe the doctor may have had was to ensure that the police and the NHS were not involved if one of the attackers accidentally poisoned himself.
The source said Borishov and Petrov, who claim to be innocent tourists, could have met the other two suspects after registering at the City Stay Hotel in Bow, East London.
The couple took a train from Waterloo to Salisbury, Wiltshire, where Novichok was daubed at the door of the house where Skripal and his daughter, Yulia, lived.
It is believed that the poisoners met with Dr. Novichok and another accomplice of the embassy after staying at this hotel in East London
Officials are believed to know the real identities of Petrov and Borishov, who are believed to use assumed names
The source said it is believed that they might have met a driver who worked at the embassy and that the doctor may have been in a supervisory role.
Other informants say that the police know the true identity of Petrov and Borishov, adding that hundreds of officers have been "tracking CCTV" with a "facial recognition software" that could compare them with the passports used in the years prior to the attack.
Former spy Sergei, 67, and his 34-year-old daughter, Yulia, recovered completely after the poisoning.
CCTV shows Boshirov and Petrov on Fisherton Road in Salisbury at 1:05 p.m. on March 4
But Dawn Sturgess, 44, died after she drove Novichok who was thrown in a perfume bottle.
A spokesman said the Metropolitan Police is not prepared to respond to speculation related to an ongoing investigation.
He added that the force continues to appeal to the public to obtain information related to the two men while they were in the United Kingdom from March 2 to March 4.
Anyone who knows or has seen them can call the police confidentially on 0800 789 321 or email firstname.lastname@example.org