Dawn Sturgess, 44, died and her partner Charlie Rowley, 45, is still in a critical condition after the couple fell ill in Amesbury, Wiltshire, last Saturday.
The murder investigation is the second major probe involving the nerve agent this year, following the case of Sergei and Yulia Skripal who were left critically ill in March. Both have recovered.
Giving the latest update on the investigation on Monday, Assistant Commissioner Neil Basu said: “The investigation is being led by detectives from the UK’s counter-terrorism poliocing network, and they are unable to say at this moment whether or not the nerve agent found in this incident is linked to the attack on Sergei and Yulia Skripal, however this remains our main line of inquiry.
“Our focus and priority at this time is to indentify and locate any container that we believe may be the source of the contamination. In the four months since the Skripals and Nick Bailey were posioned, no other people besides Dawn and Charlie have presented with symptoms. But their reaction was so severe it resulted in Dawn’s death and Charlie being critically ill.
“This means they must have had a high dose and our hypotheseis is that they must have handled a container that we are now seeking.”
A red Ford Transit van in which Mr Rowley was a passenger on June 30 has been recovered and sent for testing at the Government laboratory at Porton Down.
Three other men who also travelled in the van have been tested and show no signs of having been exposed to Novichok, Mr Basu said.
Praising the “tremendous stoicism” of people of Wiltshire, he told journalists outside Scotland Yard that 21 people have raised concerns about being exposed but have been given the all clear.
The government’s Cobra emergencies committee was due to meet at 1pm on Monday to discuss the case.