Police have spoken briefly to novichok victim Charlie Rowley as they investigate how he came into contact with the nerve agent.
Mr Rowley was left in a critical condition and Dawn Sturgess was killed after they were poisoned by the substance.
Officers have been piecing together movements of the pair in Salisbury and Amesbury before they were taken ill on Saturday 30 June.
In a statement, Scotland Yard said: "Officers from the investigation team have spoken briefly to Charlie and will be looking to further speak with him in the coming days as they continue to try and establish how he and Dawn came to be contaminated with the nerve agent.
"Any contact officers have with Charlie will be done in close consultation with the hospital and his doctors. We will not be providing further commentary around our contact with Charlie."
The UK's top anti-terror police office said on Wednesday that he could not guarantee those responsible for the poisonings will be caught.
Assistant Comissioner Neil Basu said it was still not possible to definitively link the alleged murder of Ms Sturgess and poisoning of Mr Rowley with the attack on Sergei and Yulia Skripal in Salisbury in March.
But he said he found it highly unlikely there was not a link to the attack, which the UK blames on Russia.
AC Basu also said reiterated he could not guarantee that Wiltshire was 100% safe from novichok.
He said: "In March this year, we launched an investigation after Sergei and Yulia Skripal were both poisoned with the nerve agent novichok.
"I would love to be able to stand here and say how we have identified and caught those responsible and how we are absolutely certain there are no traces of nerve agent left anywhere in the county.
"The brutal reality, however, is that I cannot offer you any such assurances or guarantees at this time.
"Late on Tuesday last week, we received that dreadful news that two more people – Dawn Sturgess and Charlie Rowley – had fallen ill as a result of exposure to novichok.
"The role of Counter Terrorism Policing is to now investigate these two incidents – the latest of which is being treated as murder and to try to identify and bring those responsible to justice.
"At this stage, we cannot say with certainty that both the incident in March and this latest incident are linked. Clearly, this is our main line of enquiry, but our investigation must be led by the evidence available and the facts alone.
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"I would need a forensic link to be definitive, but this is a very rare substance banned by the international community and for there to be two separate distinct incidents in one, small English county is implausible to say the least."
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