Former officials and legal experts say that Clinton’s testimony will likely be the final puzzle piece for federal prosecutors and FBI investigators as they decide whether to file any charges over her use of a personal email server.

“This certainly sends the signal that they are nearing an end to their investigation,” said Steven Levin, a former federal prosecutor and current partner at the law firm Levin & Curlett.

Though FBI officials have said Clinton is not the target of the probe, they would want to be as prepared as possible before going in to interview her, Levin said.

“Typically, the way we structure investigations when I was a federal prosecutor is that we would seek to interview the target last,” he said.

“As you begin to interview people who are extremely close to the target of an investigation — people who are considered confidantes … you typically interview those people towards the final stages of the investigation,” he added. “So that way if they tell you something that is contrary to something you’ve already learned, you can immediately challenge them on that information.”