Sweeping changes to Ukraine’s top law enforcement agency ordered by President Volodymyr Zelenskiy are set to derail a series of long-running criminal investigations, including two related to U.S. President Donald Trump’s former campaign chairman, Paul Manafort, three current and former Ukrainian prosecutors told Reuters.

The reorganization, which includes fresh leadership for the agency and mandatory skills testing for prosecutors, represents an upheaval of Ukraine’s General Prosecutor’s Office (GPO), one of the most powerful bodies in the country, and one that has long been the target of criticism.

Zelenskiy, who came to power in May, has said the makeover is essential because the office is widely distrusted by Ukrainians and has been seen as a political tool for the well-connected to punish their enemies.