Moscow (CNN) Russia has declared it will reopen the closed city where Russian scientists say chemical weapons and nerve agents, including the Novichok substance used in the Salisbury chemical attack, were made.

According to UK officials , the nerve agent used in the attempted murder of ex-spy Sergei Skripal and his daughter Yulia in the English city of Salisbury in March was created at a facility in Shikhany, in the lower Saratov region of Russia.

Russia has a system of closed cities that can be traced back to the Cold War. They once included strategic ports and cities with military bases that were closed off to foreigners, such as Kaliningrad and Vladivostok. They also included sites involved in the development and testing of nuclear weaponry, much like the United States had closed cities in World War II during the Manhattan Project to develop the first atomic bomb.

Russia still has cities that are part of the nuclear weapons complex, or as in the case of Shikhany, were involved in other classified research and development programs. They are largely off-limits to foreigners and access is tightly restricted.

The chemical weapons research center in Shikhany, around 600 miles south of Moscow, was once the centerpiece of Soviet nerve agent development and the place where the British believe the chemical Novichok used to poison the Skripals was made.

Former Russian spy Sergei Skripal and his daughter, Yulia Skripal, at a restaurant in Salisbury.

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