Over 300 ex-national security and foreign policy officials signed a Friday letter that called the growing controversy involving President Trump and Ukraine a “profound national security concern,” NBC News reports. The letter, signed by former officials from both the Obama and George W. Bush administrations, claims Trump’s alleged pressuring of Ukrainian officials to investigate corruption accusations against former Vice President Joe Biden and his son is an “unconscionable abuse of power.” “It also would represent an effort to subordinate America’s national interests—and those of our closest allies and partners—to the President’s personal political interest,” the letter read. The letter’s authors also praised the formal impeachment inquiry that House Democrats announced earlier this week, and said it was “imperative” and could “ascertain additional facts” about the matter.

A whistleblower complaint first raised concerns about a July 25 call between Trump and Ukrainian President Volodymyr Zelensky. The subsequent release of a memo about the call and the complaint itself revealed that Trump wanted Zelensky to coordinate with his personal attorney, Rudy Giuliani, and Attorney General Bill Barr on the investigation.