U.S. Sen. Dianne Feinstein is alleging the CIA may have violated the U.S. Constitution by spying on the staff of the Senate Intelligence Committee.

Feinstein, D-Calif., who chairs the committee and has been a longtime supporter of the CIA and other intelligence agencies, made a dramatic speech on the floor of the U.S. Senate, accusing the CIA of possible criminal activity in its attempts to obstruct the committee’s investigation into the agency’s use of torture in the aftermath of the 9/11 terrorist attacks.

CIA Director John Brennan has dismissed the charges as “beyond the scope of reason” and insists the CIA has broken no laws.

CISAC Co-Director Amy Zegart, an intelligence expert, tells Politico: “This is the most serious feud since the Intelligence committees were established."

Zegart, a former National Security Council staffer, answers questions about the dispute: 

Brennan said Feinstein's accusations of the CIA possibly violating the U.S. Constitution are "beyond the scope of reason." What's going on between these two?

This is a longstanding and before-now private, bitter fight over how the Senate Intelligence Committee is investigating the CIA's past detention and interrogation programs. The current controversy is about whether congressional staff illegally acquired CIA documents during the course of their investigation and whether the CIA illegally spied on those congressional staffers.

But the deeper fight is really about how history will record and judge the Bush administration's detention and interrogation policies. It's a complex battle that is tinged with politics. The Senate investigation began years ago as a bipartisan effort to examine these policies and their efficacy. Eventually, though, Republican members of the committee felt they could no longer participate. So we're now in a bizarre world where Democratic senators on the Intelligence Committee are feuding with a Democratic administration about investigating controversial policies practices that occurred under a Republican predecessor.

Feinstein calls the crisis a defining moment for the agency. Is it?

This is an extraordinary moment for CIA-congressional relations and the worst feud since the Church Committee investigation of CIA abuses in the 1970s. Oversight is always a delicate dance between intelligence officials who believe strongly in the importance of secrecy for carrying out their mission and lawmakers who believe strongly in the value of transparency to maintain the public trust. Intelligence officials never freely volunteer details of their agencies' activities. It's Congress's job to probe, to ask tough questions and demand good answers. 

But this is different than the usual tension between intelligence overseers and agencies. Feinstein isn't accusing the CIA of stonewalling or not telling the full truth. She is accusing the agency of spying on and intimidating her staff. That's totally out of bounds. For Feinstein, this conflict is Constitutional – raising serious questions about separation of powers and Congress's ability to check the executive's most powerful agencies. It is institutional, calling into question the role of her committee in particular. And it is personal. She is claiming that her own staff has been referred to the Justice Department for possible criminal prosecution.

How serious is this for CIA Director Brennan, who vehemently denies the accusations and says no computers were hacked. 

The CIA is in big-time trouble. We may never know what actually happened in a basement room in Virginia where congressional staffers were reading millions of pages of CIA documents on computers. What matters more now is the political narrative. Ever since Edward Snowden – the former contractor for the National Security Agency – started releasing documents last June, the political narrative has been that U.S. intelligence agencies spy on and lie to just about everyone.

Much of this narrative is woefully distorted and just plain wrong. NSA is not out there listening to your calls with grandma. And there is still no evidence that NSA programs violated the law. But the narrative of distrust is powerful. Which is why Sen. Feinstein's charges against the CIA -- coming from one of the intelligence community's biggest defenders -- is so devastating.