Britain’s military laboratory has not established whether the nerve agent that poisoned a Russian former double agent and his daughter came from Russia, the laboratory’s chief executive said on Tuesday.

“We have not verified the precise source,” the executive, Gary Aitkenhead, told Sky News. Mr. Aitkenhead said his laboratory, the Defense Science and Technology Laboratory, had confirmed that the poison was Novichok, or from its family of nerve agents, and that it was of military grade.

“We provided the scientific information to the government, who have then used a number of other sources to piece together the conclusions they have come to,” Mr. Aitkenhead said. He said it was “not our job to say where it actually was manufactured.”

The British authorities have blamed Russia for the March 4 poisoning, with Foreign Minister Boris Johnson suggesting it was “overwhelmingly likely” that President Vladimir V. Putin had ordered the attack. Last week, investigators said the nerve agent had been smeared on the front door of the home of the former spy, Sergei V. Skripal, in Salisbury, England.