FBI director Christopher Wray is prepared to issue a rebuttal if the White House releases Rep. Devin Nunes's classified memo alleging inappropriate surveillance of the Trump campaign by the FBI and Justice Department, according to CBS News senior national security analyst Fran Townsend. The FBI issued a statement Wednesday that they have "grave concerns" about the memo and the "material omissions of fact that fundamentally impact the memo's accuracy."

Townsend, who served as homeland security adviser to President George W. Bush, told "CBS This Morning" she believes the FBI is worried about both the accuracy of the memo's contents and what it may reveal about their sources and methods.

"I think we have to remember the Nunes memo is an advocacy piece. It's not a fact piece. This is Chairman Nunes' summary of what he believes the abuses are. For that reason, it's one-sided," Townsend said.

Townsend, who spent 13 years at the Justice Department, said it's simply "not possible" for one partisan actor to push through a FISA warrant or to obtain one based on a single piece of evidence.

"There's multiple internal reviews in the FBI, there's a legal review at the Justice Department, it goes to the attorney general, or in this case, the deputy who reviews it and then it goes to an independent federal judge who looks at it. No FISA warrant relies on a single piece of evidence. So if the allegation from Chairman Nunes is that they relied solely on the Steele dossier, that's not possible. It never happens," she said.

Earlier this week, FBI Deputy Director Andrew McCabe stepped down amid mounting pressure from the FBI director.

Townsend also addressed a Washington Post report about an internal Justice Department probe focusing on McCabe's slow handling of a batch of Hillary Clinton's emails uncovered in the late stages of the 2016 campaign.

"My understanding here from sources inside the FBI was initially they believe there might have been as many as 6,000 e-mails some of which may have been classified and there would have had to been an entire process to review that," Townsend said. "I think we shouldn't attribute malice where it might have just been bureaucratic caution."