Glenn Greenwald, the Guardian columnist who has broken a series of stories about the National Security Agency's surveillance powers, said Sunday that even low-level NSA analysts have the ability to search through private communications.

Greenwald's comments defended bombshell revelations from former NSA contractor Edward Snowden at the time, which have since come under scrutiny by intelligence analysts. Greenwald is set to testify before Congress on Wednesday, along with other NSA surveillance critics and analysts.

Greenwald dared NSA officials to dispute the claims when testifying this week. Though he said that there are "legal constraints" like the FISA court's approval on NSA surveillance, this type of surveillance goes through little supervision.

"There are legal constraints for how you can spy on Americans. You can’t target them without going to the FISA court. But these systems allow analysts to listen to whatever emails they want, whatever telephone calls, browsing histories, Microsoft Word documents," Greenwald said on ABC's "This Week" with George Stephanopoulos.

"It’s an incredibly powerful and invasive tool, exactly of the type that Mr. Snowden described. NSA officials are going to be testifying before the Senate on Wednesday, and I defy them to deny that these programs work exactly as I just said."

Greenwald said he would publish a story in the coming week that details the process. He claimed that an NSA analyst simply needs to enter an email or IP address, upon which a search of the database lets them listen to the calls or read the emails of "everything the NSA has stored."

In a subsequent interview on the show, Sen. Saxby Chambliss (R-Ga.), the ranking Republican on the Senate Intelligence Committee, said he would be "shocked" if those claims were true.

He did, however, seem to leave the door open to "accidental" abuse.

"I was back out at NSA just last week, spent a couple hours out there with high and low level NSA officials. And what I have been assured of is that there is no capability at NSA for anyone without a court order to listen to any telephone conversation or to monitor any email.

In fact, we don't monitor emails. That's what kind of assures me that what the reporting is is not correct, because no emails are monitored now. They used to be, but that stopped two or three years ago. So I feel confident that there may have been some abuse, but if it was it was pure accidental."