Nerve Agent That Poisoned Four People in the U.K. Could Remain Active for 50 Years, Police Say

British police said that novichok, the powerful nerve agent that killed one person and left another critically ill in southwestern England, could be active for up to 50 years if it remains in a container.

Head of counter-terrorism of the Metropolitan Police, Neil Basu, made the comments while holding a public meeting in the English town of Amesbury Tuesday, the Guardian reports.

Police have yet to establish how the couple, 44-year-old Dawn Sturgess and 45-year-old Charlie Rowley, came into contact with the nerve agent. Police believe they touched a contaminated item that has not been found.

Basu was asked if the military-grade poison could be in a landfill site.

“If it is sealed in a container and it was in a landfill site it would effectively be safe because it would not be touched by anyone and it would last, probably, I’ve been told by scientists, 50 years,” he said.

When asked if Basu was looking for a “needle in a haystack” and he said, “That’s why we need witnesses or intelligence,” Sky News reports.

Read More: ‘There Will Be Severe Pain.’ What to Know About the New Novichok Nerve Agent Poisoning in the U.K.

Novichok first made headlines in March when it was used to poison former Russian spy Sergei Skripal and his daughter Yulia, who both became critically ill. Skripal, 67, and his 33-year-old daughter survived the attack in the English town of Salisbury, which the British government blames on Russia. Moscow strongly denies any involvement.

Last week, Sturgess and Rowley were found collapsed at a residential building in Amesbury, Wiltshire, just eight miles north of Salisbury, where the Skripals were poisoned. Tests confirmed that the couple were exposed to the same nerve agent that poisoned the Skripals. Sturgess died in hospital on July 8 while Rowly remains in critical condition.

The British government is working under the assumption that the couple’s exposure is a consequence of the first attack.

Basu said Tuesday police have found no forensic link between the two cases, and it was possible that a link would never be established, the Guardian reports. However, he said that it was “implausible” for there to be no connection between two incidents.

“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,” Basu added.