The raid was conducted on December 4 over suspicions that the NGO had been involved in funding a banned publication, Novy Petersburg. The newspaper was earlier closed after two official warnings about its "extremist" activities.
"We are deeply concerned about a raid by Russian law enforcement officers on the research center operated by Memorial in St. Petersburg last week," State Department spokesman Sean McCormack said.
He added that "Memorial is a respected non-governmental organization dedicated to remembering the victims of totalitarian repression and the historical documents and information in its archives are invaluable sources for historians and social scientists worldwide."
The State Department urged Russian authorities "to ensure the speedy and safe return of all seized equipment and archival material."
McCormack also said the action against Memorial was "not an isolated instance of pressure against freedom of association and expression in Russia."
"The Memorial Society and its Centre know absolutely nothing about the newspaper publication, the ostensible pretext for the search," the group said in a statement, adding that, "It would seem that the publication is no more than an excuse to search the Memorial offices."
"The confiscated discs contain databases with biographical details of tens of thousands of victims of the Stalinist repressions. It has taken Memorial 20 years to collect them," the statement also read, going on to say that it had "warned the St Petersburg Office of the Prosecutor General that this material is of immense academic value and that the total responsibility for its safety must be borne by the Prosecutor General's Office."
The non-profit Open Democracy Russia website wrote that, "Some Russian human rights lawyers are maintaining that the real reason goes back to Memorial's screening a month ago of the film Revolt: the Litvinenko Affair, which has been banned in Russia."