In this occasional series, we will bring you up to speed on the biggest national security stories of the week.

On Wednesday, The Washington Post reported that the special counsel overseeing the Russia investigation is widening its probe to include a look at whether President Trump attempted to obstruct justice. In a sign of the investigation's new focus, Special Counsel Robert S. Mueller III’s team is interviewing a number of intelligence officials who we know had conversations with Trump about the Russia investigation.

Trump seemed to confirm that he is under investigation on Friday morning when he criticized the special counsel's probe in a tweet.

"I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt," Trump said.

The fact that the president is being investigated is an astonishing development on its own. But what's also striking is that Trump has gone to great lengths to prove that he wasn't under investigation. Former FBI Director James B. Comey has testified that the president asked him to publicly clear his name, and the director declined. Once the president fired Comey, though, things appear to have changed.

Here's what you need to know to understand this fast-moving story:

Why is this important?

At this point, the FBI's investigation has lasted for almost a year and focused on whether there was any coordination between Trump's campaign and Russia during the 2016 race, as well as the Kremlin's alleged interference in the 2016 election.

Trump has repeatedly sworn that he is personally not under investigation, and Comey assured him that he wasn't. Now, that's no longer the case. The special counsel's interviews with the senior officials will likely focus on Trump's firing of Comey and what motivated him.

Who is the special counsel planning to talk to and why do they matter?

Mueller's investigators plan to speak with Daniel Coats, the director of national intelligence, and Mike Rogers, the head of the NSA, according to five people briefed on the interview requests.

Coats told associates that Trump asked him if he could intervene with Comey to get the FBI to back off its focus on former national security adviser Michael Flynn, according to officials. Coats ultimately decided that interfering would be inappropriate.

That particular interaction suggests that Trump hoped to get top officials to push back on the bureau's probe and could be relevant to investigators who want to understand the conversations the president was having regarding the FBI probe.

Investigators might be interested in Rogers because Trump at one point asked him to issue a public statement denying the existence of any evidence of coordination between his campaign and Russia, according to officials. This, too, could be a sign of the president acting inappropriately.

Mueller also hopes to talk with former NSA deputy director Richard Ledgett, who left the agency recently but wrote an internal NSA memo documenting the president's call with Rogers. It is unclear who the FBI has already questioned.

Can't the White House just prevent the officials from talking?

Potentially. The White House and the president in theory can try to invoke executive privilege, which protects the private deliberations of the executive branch.

During the Watergate scandal, former president Richard Nixon attempted to use executive privilege to conceal information. However, the Supreme Court ruled that officials cannot invoke executive privilege to suppress evidence in a criminal probe.

Can the president really be charged?

Experts say the Justice Department has held that it would not be appropriate to indict a sitting president. The responsibility would fall to Congress to review any findings of misconduct.

Will Trump fire Mueller, the same way he dismissed Comey? 

The president can be hard to predict, and one of his friends raised the possibility this week.

During an appearance on PBS NewsHour, Christopher Ruddy said Trump was "weighing that option" but thought it would be a "significant mistake" if the president were to take that action. Republicans have tried to tamp down speculation of the president going that far.

“I think the best thing to do is to let Robert Mueller do his job,” House Speaker Paul D. Ryan (R-Wis.) said. “I think the best vindication for the president is to let this investigation go on independently.”

Deputy Attorney General Rod J. Rosenstein said only he has the power to fire Mueller, and in his testimony this week he said he has no cause to fire him.