Those attacks have been known to the American government for more than a year, but kept highly classified. Symantec Corporation, which markets systems that detect and combat malware, issued a report about Dragonfly in October, but stopped short of naming the Russians.

Among the targets was the Wolf Creek Nuclear Operating Corporation, which runs a nuclear power plant near Burlington, Kan., according to security responders and a joint report issued by the Department of Homeland Security and the F.B.I. last June.

Those attacks suggest Russian state-sponsored hackers have been mapping out Western industrial, power and nuclear facilities, perhaps in preparation for eventual sabotage. There is no evidence that Russian hackers were able to jump from the operator’s corporate networks into the production networks that control plant operations, but forensic investigators and government officials believe the attacks were designed to look for ways to make that jump.

The joint statement responding to the March 4 poisoning attack in Salisbury, England, came a day after Mrs. May expelled 23 Russian diplomats and announced other measures in retaliation. Sergei V. Skripal, the former Russian spy, and his daughter, Yulia, were left in a coma because of the attack, which British investigators attributed to Novichok, a potent nerve agent developed in the Soviet Union in the 1970s and ’80s.

“This use of a military-grade nerve agent, of a type developed by Russia, constitutes the first offensive use of a nerve agent in Europe since the Second World War,” the statement by the four allies said. “It is an assault on the United Kingdom’s sovereignty and any such use by a state party is a clear violation of the Chemical Weapons Convention and a breach of international law. It threatens the security of us all.”

Although it did not include concrete steps in response, the joint statement was a notable display of unanimity among countries that have taken varying stances toward Russia in recent months. Mrs. May has sought in recent days to marshal criticism of Russia, submitting statements to international organizations like the United Nations Human Rights Council.

Russia has denied responsibility for the attack. “Neither Russia nor the Soviet Union has ever pursued any programs to develop chemical warfare agent called Novichok,” Mr. Ryabkov said. “The allegations that such a program existed have been spread by people who were earlier transferred to the West and who actually emigrated not without Western governments’ assistance.”