Authorities announced that Doruk Bulut, head of the main opposition Republican People’s Party’s (CHP) branch in Istanbul’s Silivri district, was detained on Tuesday in a corruption investigation.

The investigation has focused on alleged corruption in Silivri Municipality. Bora Balcıoğlu, the CHP’s mayor of the district, was arrested last month on corruption charges.

Prosecutors said in a statement that the investigation that netted Bulut focused on alleged crimes including running a criminal organization, bribery, tender rigging and money laundering.

Based on the testimony of a suspect who collaborated with investigators, the probe found that Balcıoğlu had large sums of bribe money exchanged at various foreign exchange offices through his chief of staff, Aşkın Kaynar, and his driver, Serdar Tuna. It was also discovered that part of the laundered bribe money was sent directly to Bulut, according to a report by the Sabah newspaper on Tuesday.

The investigation, whose details were revealed by the Sabah newspaper, found that suspects in a criminal ring allegedly formed by the mayor and municipal staff were involved in several irregularities that caused losses of more than TL 21 million ($440,000) in municipal resources. The suspects are also accused of irregularities in staff recruitment, leasing procedures, zoning and permit approvals, and the sale of municipal properties.

Silivri is also home to a massive courthouse-prison complex where several CHP mayors, including former Istanbul Mayor Ekrem Imamoğlu, are held in custody following their arrests on corruption charges. Imamoğlu has faced a spate of trials, mostly stemming from corruption.

Dozens of CHP mayors and municipal bureaucrats have been detained or arrested over the past two years in investigations focusing on corruption. The majority of the allegations involve bribery in exchange for construction permits that are otherwise subject to strict regulations, as well as rigging lucrative municipal tenders.