President Trump has grown increasingly convinced that the FBI used an informant to spy on his 2016 campaign, and has now demanded a Justice Department investigation:

The back story: There has been a growing amount of speculation by conservative writers that an FBI source spied on the Trump campaign, and might have even planted a spy inside. It's now clear from multiple news reports that the FBI had an informant who talked to two campaign advisers, but not that the informant was planted on the inside. That talk is based on suspicions — not evidence.

Between the lines: This all started with House Intelligence Committee chairman Devin Nunes' fight to get the Justice Department to turn over documents about an intelligence source who helped the Russia investigation.

The Washington Post reported on this source earlier this month, and the New York Times reported last week that the informant — an American academic who teaches in Britain — met with Trump campaign advisers Carter Page and George Papadopoulos, but only after the FBI was looking into their contacts with Russia.

The Wall Street Journal identified Stefan Halper as the suspected informant on Sunday, but neither the White House nor the FBI have confirmed it. Axios' Jonathan Swan reports that Trump trade adviser Peter Navarro recommended Halper for a senior role in the Trump administration.

Beyond that, the columns read a lot into what the source could have been doing.

Here's what the Wall Street Journal's Kimberley Strassel wrote in a May 10 column:

"When government agencies refer to sources, they mean people who appear to be average citizens but use their profession or contacts to spy for the agency. Ergo, we might take this to mean that the FBI secretly had a person on the payroll who used his or her non-FBI credentials to interact in some capacity with the Trump campaign.
This would amount to spying, and it is hugely disconcerting."

National Review's Andrew McCarthy got more specific in a May 12 piece, noting that Glenn Simpson, the co-founder of Fusion GPS, testified to a Senate committee that former British spy Christopher Steele told him the FBI had a "human source" inside the Trump campaign —and then tried to walk it back. McCarthy believes Simpson got it right the first time.

And when he appeared on Fox and Friends last week, McCarthy went farther: "There's probably no doubt that they had at least one confidential informant in the campaign." That's the line Trump quoted in a tweet declaring the possible spying was "bigger than Watergate."

The bottom line: We still have no evidence that the FBI actually planted a source inside the Trump campaign, and the reports from the New York Times and CNN concluded that it didn't. If there's any evidence that contradicts that, we won't know more unless it surfaces from the Justice Department inspector general's investigation into whether there was "any impropriety or political motivation" in the FBI's Russia investigation.

The other way we could learn more is if Nunes gets his information on the source. But if he discloses any of it, intelligence officials are warning that he could put the source in danger, per the Washington Post.

Until that gets resolved, this story is likely to be stuck in an uncomfortable limbo, with no evidence to back up the suspicions of questionable FBI surveillance — but no way to put them to rest, either.

This story has been updated to include Trump's demand for an investigation and more reporting on the FBI source.

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