Former Director of National Intelligence James Clapper said Thursday that the whistleblower’s complaint that prompted the House of Representatives to launch an impeachment inquiry into President Donald Trump’s actions was the best documented he had ever seen.

“I’ve seen a lot of whistleblower complaints during the time I was DNI, and this had to be the best composed, best written, best documented such complaint I’ve ever seen,” Clapper told CNN’s Jake Tapper in an interview.

It was “obviously done by a seasoned, mature, capable intelligence professional, written like an intelligence document,” he continued, “And it’s very clear. He points out what he knows — he or she, whoever it is. So I thought it was very compelling.”

Clapper, who served as director of national intelligence for nearly seven years during the Obama administration, previously served as director of the Defense Intelligence Agency during the Clinton administration and director of the National Geospatial-Intelligence Agency for five years during George W. Bush’s presidency.

The interview came just a few hours after one of Clapper’s successors at the Office of the Director of National Intelligence, Joseph Maguire, testified before the House Intelligence Committee about the whistleblower complaint.

Earlier Thursday, The New York Times reported that the whistleblower who wrote the complaint, which focuses on the events surrounding a call between President Donald Trump and Ukrainian President Volodymyr Zelensky, was a CIA officer who’d been detailed to the White House.

The President on Thursday implied that whoever leaked information to the whistleblower ought to be treated like a spy, which Clapper and others have said raises concerns of retaliation.

“At a minimum, it is witness retaliation,” Clapper said. “It actually conflicts with the spirit of the intelligence Community Whistleblower Protection Act, because one of the purposes is to insulate complainants from retaliation. Clearly, this is retaliation threatened in the worst way.”