By Mick Krever, CNN

Glenn Greenwald on Wednesday said that the head of the U.S. National Security Agency, General Keith Alexander, did not offer “any evidence” when he told Congress on Tuesday that the NSA did not collect data on millions of citizens in Europe.

“Notice what he didn’t offer, which is any evidence for the truth of what he’s is saying,” Greenwald said in an interview with CNN’s Christiane Amanpour.

Greenwald is the activist and journalist who broke the story of NSA snooping and has a trove a leaked documents from former intelligence staffer Edward Snowden.

“This, remember, is an agency that is extremely beleaguered, in the middle of a very intense scandal both at home and abroad,” he said. “It is an agency whose top officials have a record of lying to the Congress and to the American people through the media, including General Alexander.”

On Tuesday, General Alexander went before Congress and batted down media reports that the NSA had collected data on tens of millions of phone calls in a single month in France and Spain.

“The assertions,” General Alexander said, “are completely false… But both they and the person who stole the classified data did not understand what they were looking at.”

Greenwald called those remarks “accusations made without evidence,” and said that he was “astonished” to see journalists go on television Tuesday and repeat Alexander’s claims “as though they were the gospel.”

“This reporting has been going on for months, Christiane, in Germany and Brazil,” Greenwald said. The NSA has “never once denied that this reporting is accurate. Suddenly they make this assertion in the middle of this scandal, and I think some skepticism is warranted.”

As to whether he himself could have made a mistake, Greenwald did not reject the idea, but stood by his reporting.

“Journalists are human beings and all journalists and all human beings make mistakes,” he told Amanpour. “Of course it’s possible that at some point we’ll make a mistake.”

“But I gathered all the evidence today that I think is relevant to this question,” he said. “It showed exactly what the basis is. Go look at what the NSA says these documents do when they thought that they were talking in private, and the basis is very clear for why it is we reported what we did.”

UPDATE: A previous version of this article mistakenly referred to the NSA as the National Security Administration. It is the National Security Agency.