(CNN) The British government repeated its claim that Russia was responsible for the Salisbury poisoning after UK scientists said they were unable to determine where the substance used to attack former Russian spy Sergei Skripal and his daughter Yulia was made.

Skripal, 66, and Yulia, 33, were poisoned on March 4 after being exposed to a nerve agent in Salisbury, England. The pair were found slumped on a bench in an outdoor shopping complex after the attack with no visible injuries, according to police. The nerve agent had been placed on Sergei Skripal's front door, police said.

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Speaking to Sky News on Tuesday, Gary Aitkenhead, chief executive of Porton Down laboratory, said the poisonous substance used to carry out the attack was the military-grade nerve agent Novichok but said scientists did not identify where it was manufactured.

"We have not identified the precise source, but we have provided the scientific info to government, who have then used a number of other sources to piece together the conclusions you have come to," Aitkenhead said.

He added: "It is our job to provide the scientific evidence of what this particular nerve agent is. We identified that it is from this particular family and that it is a military grade, but it is not our job to say where it was manufactured."

Police officers stand outside former Russian spy Sergei Skripal's home.

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