Turkey: Three ministers resign amid widening corruption probe GAZIANTEP, Turkey -- Three senior ministers in Turkey's Cabinet resigned Wednesday as a secret 14-month corruption investigation of tender-rigging and bribery intensified, threatening to topple the ruling Justice and Development Party (AKP). The three-pronged graft probe presents the most serious challenge to Prime Minister Recep Tayyip Erdogan government's legitimacy since anti-government protests last summer. The investigation has so far netted millions of dollars in cash stashed away in shoe boxes, exposed sanctions-breaking deals with Iran and revealed bribes paid to allow construction in protected areas. Outrage on the streets of Turkey has morphed into protest, with thousands taking to the streets over the weekend after a series of dawn raids last week that targeted high-profile businessmen, the mayor of Istanbul's Fatih district, bureaucrats and the sons of three prominent ministers. Turkish Economy Minister Zafer Caglayan, Interior Minister Muammer Guler and Environment and Urbanization Minister Erdogan Bayraktar resigned from their posts Wednesday. Each of the ministers had a son arrested in the graft probe. Statements from both Caglayan and Guler -- who earned the title "chemical Guler" for his part in putting down anti-government protests with tear gas earlier this year -- reflected the rhetoric of Erdogan, who has framed the corruption probe as an international conspiracy led by a "dark alliance" of "traitors and spies." Bayraktar, however, shifted the focus to the prime minister, claiming that a majority of the construction plans in the corruption investigation were approved by Erdogan, local media reported. "I believe that the prime minister should step down as well in order to relieve the Turkish public," Bayraktar said, according to the newspaper Today's Zaman. In the last week, Erdogan has overseen the firing of a reported 500 police officers investigating the corruption allegations. They include Istanbul Police Chief Huseyin Capkin and Nazmi Ardic, the head of the organized crime branch. His response has been complete denial of any wrongdoing on the part of the ministers or their families. He has not acknowledged or said a single word about corruption," said Faruk Logulu, vice chairman of the main opposition Republican People's Party (CHP). Instead he began threatening prosecutors and saying it was the work of gangs. This is entirely inappropriate and contravenes due process. The frenzied purge has prompted allegations of political meddling and raised fear that Erdogan will bulldoze over the investigation. "There is growing concern that the actions of the government could be considered as interference with the ongoing investigation," said Oya Ozarslan from Transparency International's Turkey chapter. The graft probe has widely been interpreted as part of a rift between the AKP and supporters of the cleric Fethullah Gulen. Gulen's supporters are believed to have penetrated the Turkish police and judiciary. The organization was a staunch ally of the AKP for much of the last decade, mobilizing its supporters and helping the AKP win three consecutive elections. However, revelations that the country's intelligence services may have been profiling Gulen's supporters, and Erdogan's increasing authoritarianism and moves against the movement's economic interests -- notably private schools -- have soured the relationship.