Turkish President Recep Tayyip Erdogan has been cracking down on followers of his archrival, Fethullah Gulen, for years. Now he’s going after the US-based cleric himself, with unpredictable ramifications for Ankara’s already fraught relationship with Washington.

The Turkish Embassy in the United States hired British law firm Amsterdam and Partners in October to weigh legal action against Gulen, the brains behind more than 100 highly successful charter schools in the United States and thousands more around the world. The law firm filed suit on behalf of three Turkish defendants in a Pennsylvania court Dec. 7, alleging that the Muslim cleric ordered his followers in the Turkish judicial system to unlawfully arrest and detain members of a rival Islamic movement in 2009, according to Reuters.

The lawsuit appears aimed at getting the United States to send Gulen back to Turkey, where he stands accused of running a parallel state and seeking to topple the elected government. A Turkish court issued a warrant for his arrest earlier this month on charges of operating an armed terror group; the Turkish Embassy’s $50,000-per-month contract with Amsterdam explicitly calls on the firm to “provide legal advice and representation related to the extradition of persons from the United States to Turkey as required by treaty.”

The fight between Erdogan and Gulen, two former Islamist allies who had a falling out several years ago, now risks embroiling Congress and the Obama administration as well since the lobbying contract with Amsterdam “may also involve lobbying before state and/or federal government entities.“ Gulen’s supporters have spent hundreds of thousands of dollars on more than 200 lavish trips to Turkey for US lawmakers and their staff and supporting presidential candidates or organizations they are tied to, notably donations to Democratic front-runner Hillary Clinton’s Clinton Foundation.