Co., which in the fall of 1916 had begun surface mining at Kukruse and Järve, respectively, also terminated their mining activities in 1917. In February 1918, the area surrounding the oil shale basin in northeast Estonia was occupied by German troops. During this occupation, mining activities were carried out at Pavandu by the German company Internationales Baukonsortium (), including sending oil shale to Germany for research and experimentation. This work used a retort constructed by Julius Pintsch AG, known as a Pintsch generator. In late 1918, German forces left Estonia, by which time no more than a single trainload of oil shale had been mined and sent to Germany. Developments in interwar Estonia After Estonia gained independence, the state owned oil shale enterprise, Riigi Põlevkivitööstus (), was established as a department of the Ministry for Trade and Industry on 24 November 1918. The enterprise, later named Esimene Eesti Põlevkivitööstus (), was the predecessor of Viru Keemia Grupp, one of the current shale oil producers in Estonia. It took over the existing Pavandu open-pit mine, and opened new mines at Vanamõisa (1919), Kukruse (1920), and Käva (1924). Also, several private investors, including investors from abroad, initiated oil shale industries in Estonia by opening mines at Kiviõli (1922), Küttejõu (1925), Ubja (1926), Viivikonna (1936), and Kohtla (1937). Pavandu mine was closed in 1927 and Vanamõisa mine was closed in 1931. While in 1918 only 16 tonnes and in 1919 only 9,631 tonnes of oil shale were mined, in 1937 the annual output exceeded one million tonnes. In 1940, the annual output reached 1,891,674 tonnes. Initially, oil shale was used primarily in the cement industry, but also for firing locomotive furnaces and as a household fuel. The first major industrial consumers of oil shale were cement factories in Kunda and Aseri. By 1925, all locomotives in Estonia were powered by oil shale. Shale oil production started in Estonia in 1921 when Riigi Põlevkivitööstus built 14 experimental oil shale processing retorts in Kohtla-Järve. These vertical retorts used the method developed by Julius Pintsch AG that would later evolve into the current Kiviter processing technology. Along with the shale oil extraction plant, an oil shale research laboratory was founded in 1921. Following the experimental retorts, the first commercial shale oil plant was put into operation on 24 December 1924. The German-owned company Eesti Kiviõli (, , predecessor of Kiviõli Keemiatööstus), affiliated with G. Scheel &