Early in 1940, the Paris Group decided on theoretical grounds that heavy water would be an ideal moderator for how they intended to use it. They asked the French Minister of Armaments to obtain as much heavy water as possible from the only source, the large Norsk Hydro hydroelectric station at Vemork in Norway. The French then discovered Germany had already offered to purchase the entire stock of Norwegian heavy water, indicating that Germany might also be researching an atomic bomb. The French told the Norwegian government of the possible military significance of heavy water. Norway gave the entire stock of to a "Deuxième Bureau" agent, who secretly brought it to France just before Germany invaded Norway in April 1940. On 19 June 1940, following the German invasion of France, it was shipped to England by the Earl of Suffolk and Major Ardale Golding, aboard the steamer . The heavy water, valued at £22,000, was initially kept at HM Prison Wormwood Scrubs, and was later secretly stored in the library at Windsor Castle. The Paris Group moved to Cambridge, with the exception of Joliot-Curie, who remained in France and became active in the French Resistance.