Gordon Sondland, the U.S. ambassador to the European Union, told House impeachment investigators Thursday that President Donald Trump put his personal lawyer Rudy Giuliani in charge of American foreign policy involving Ukraine. Sondland’s testimony, however, was delayed because Trump had demanded that Sondland not participate in the House’s impeachment inquiry despite being subpoenaed.

Marie Yovanovitch, former U.S. ambassador to Ukraine who was ousted by Trump and is currently on leave, also broke with the administration and testified last week. Though Trump has sought to invoke executive privilege over employees within the executive branch, former Department of Justice Lawyer Michael Zeldin said Trump cannot prevent them from testifying.

“The president has the right, if the standards are met, to assert executive privilege with respect to advice that they may have provided to the president in conversations in the deliberative sense," Zeldin said Thursday in an interview with Boston Public Radio. "But beyond that, they are free to testify.”

Trump's attempt to prevent his aides and staff from testifying is not unusual, Zeldin said, adding that both Richard Nixon and Bill Clinton tried to block aides from working with Congress during their impeachment investigations. Trump, however, differs in his absolute refusal to cooperate with the investigation.

“Usually what happens is each side stands back for a second, and they reach some sort of compromise,” Zeldin said. “Here, the president is unwilling to reach compromises, it appears, and the individuals are acting out of their own sense of right and wrong.”

Zeldin is is a legal analyst for CNN.