This post has been updated with the latest news

Was someone spying on the Trump campaign for the FBI?

That's what the president and his allies think, and it's become Trump's latest rallying cry in trying to discredit and even end the investigation into whether the Trump campaign helped Russia interfere in the 2016 election.

By now, it's not unusual for the president and his allies in Congress to wage war against officials in the Russia investigation. His House Republican allies have even drafted articles of impeachment against the person who set up the special counsel, Deputy Attorney General Rod J. Rosenstein.

This battle is different. The intelligence community usually keeps its head down in these attacks, but they are fearful that Trump could jeopardize their source's life by demanding the Justice Department hand over the source's identity to House Republican Intelligence Committee Chairman Devin Nunes (R-Calif.)

The Post's Philip Rucker, Robert Costa, Carol D. Leonnig and Josh Dawsey have the latest in this saga here.

And below we put together a cheat sheet, broken down by key players, for a battle involving the Russia investigation that could reverberate beyond the national security community.

Key player 1: Nunes

Who he is: The chairman of the House Intelligence Committee. He technically recused himself from the Russia investigation last year, and Republicans on the committee are technically done investigating Russian interference in the election. (They said there was interference but that it wasn't to help Trump and there was no collusion.)

Why he's key to this story: Nunes wants documents about a person who is essentially a longtime spy for the CIA and the FBI, as part of an investigation into how the Russia-Trump investigation got started.

We don't know much more about this person other than, The Post reports, she or he is a U.S. citizen who  provided information to the FBI early on about connections between Russia and the Trump campaign. The New York Times reported that a government informant met with then-Trump campaign advisers Carter Page and George Papadopoulos. The FBI later got a warrant to spy on Page.

Where the rub is: Nunes wants what the Justice Department can't give, sources tell my colleagues. The intelligence community is concerned that handing this information over could jeopardize the source's safety and further investigations.

Key player 2: Jeff Sessions

Who he is: The attorney general, a Trump campaign ally on whom the president soured after Sessions recused himself from overseeing the Russia investigation. Shortly after Sessions's recusal, his newly empowered No. 2 appointed Robert S. Mueller III to lead a special investigation.

Why he's key to this story: Nunes has threatened to hold Sessions in contempt if Sessions's department doesn't hand over the documents about this individual.

Where the rub is: A House Republican and Trump ally threatening a member of the Trump administration.

But Nunes's threat also doesn't make much sense, since Sessions doesn't oversee the Russia investigation.

Key player 3: Intelligence officials

Who they are: A conglomerate of people, ostensibly at high levels in the CIA and FBI and Justice Department, concerned about Nunes's request.

Why they're key to this story: They took their concerns about Nunes's request to Trump's chief of staff, John F. Kelly, last week, reports The Post. They were concerned that sharing information about the source could endanger a secret intelligence source.

Where the rub is: They originally won this argument. Kelly sided with the intelligence officials, and Trump sided with Kelly. It marked a rare moment where Trump was backing the Justice Department for keeping information from his allies in the House.

Until now ...

Key player 4: Trump

Who he is: The president

Why he's key to this story: Trump's position backing the Justice Department felt too precarious to be true. It's not normally what he does.

Sure enough, Trump and his allies quickly turned on this FBI informant's existence to claim that the Obama administration was spying on them.

“It looks like the Trump campaign in fact may have been surveilled,” White House adviser Kellyanne Conway said Thursday on Fox News Channel.

Where the rub is: If Trump officially changes his mind and tells the Justice Department to give documents to his House GOP ally, that could force a major showdown between congressional Republicans and Trump on one side and the Justice Department on the other. In other squabbles, the Justice Department has stood down and handed over most of the documents that congressional Republicans requested.

But this could be different. Intelligence officials are concerned that lives could be at stake in a very direct way, and were so concerned they took their worries to the White House. They don't seem willing to bend on this one.

That means Trump could dramatically escalate an already heated battle between the president and his own administration, one where lives could be at stake.