On 21 February 2010 operations were carried out in nine provinces. 49 people were detained including admirals, generals and colonels, some of them retired. On 23 February six of them were released, but seven were arrested. Another five suspects were arrested on 24 February. Further detentions and arrests followed, but at the beginning of April 2010 the number of people in pre-trial detention had dropped to nine, while 35 arrested suspects had been released, including prime suspect retired general Çetin Doğan, who had been the commander of the First Army. On 31 March and 1 April, 28 suspects in the investigation were released. A judge released 19 of the suspects due to the "existence of serious doubt regarding the crime". 9 others were released due to their "social standing." On 5 April 2010 an additional 95 people (including 86 military personnel, 70 of them on active duty), were detained in 14 provinces on the same day. Nine suspects released earlier were rearrested on 6 April. However, Istanbul's chief prosecutor, Aykut Cengiz Engin ordered the removal of prosecutors Mehmet Berk and Bilal Bayraktar from the probe as his order "any detention decision should not be taken without my approval" was not abided by. On 6 April Engin also replaced coordinating prosecutor of the investigation Süleyman Pehlivan with Mehmet Ergül. In May and June 2011 another round of interrogations followed resulting in the arrest of 15 members of the Turkish Armed Forces. The arrests are based on documents reportedly seized in February from a house that belonged to Col. Hakan Büyük's son. The new evidence consists of written documents, video files and digital material on a flash disk, and includes plans to be put into action if the coup attempt were to fail. The trial started on 16 December 2010 with the first court hearing, held in the court house of Silivri Prison. The presiding judge, Ömer Diken, was appointed only two days earlier, after the previous presiding judge was removed because of an ongoing disciplinary investigation. 187 defendants were present. Nine defendants including retired General Ergin Saygun had not appeared. None of the defendants were in pre-trial detention. After the hearing of 11 February 2011 the court decided to arrest 163 defendants. Meanwhile, the court room in Silivri Prison was extended. On 20 February 2011 an additional indictment was forwarded to Istanbul Heavy Penal Court 10. It charged 28 defendants, 15 of them in pre-trial detention in connection with documents found at the home of Colonel Hakan Büyük's son with "the attempt to remove the government of the Turkish Republic" and demanded sentences between 15 and 20 years' imprisonment.