As the Justice Department’s review into the now-debunked Russia collusion probe becomes more public, one issue likely to dominate debate is whether the FBI treated Hillary Clinton and Donald Trump differently during the 2015-16 presidential election.

Exhibit One in that debate may involve a December 2015 event that has been kept from public view for most of the last three years.

A half dozen sources with direct knowledge tell me that Clinton’s presidential campaign was given a defensive briefing from U.S. intelligence that month about a possible foreign threat.

Government officials have known for months about the briefing but were forbidden from talking about it because it was classified.

But in recent weeks congressional investigators and others were allowed to see a document where a reference to the briefing was left in an unclassified section of the memo.

My sources say the FBI was a participant in the briefing but they cannot tell me which foreign power was raised with Clinton’s team because it remains classified. Several said the threat did not involve Russia, Ukraine or China though.

FBI and Justice Department officials declined comment, and former Clinton campaign chairman John Podesta did not respond to phone and email requests for comment.

Whatever country was raised with Clinton’s campaign, the existence of the briefing itself is likely to revive concerns, at least among Republicans, that political bias caused the FBI to treat Trump and Clinton differently during the last presidential election.

While the FBI provided a generic briefing to candidate Trump about foreign threats, the bureau never gave Trump a defensive briefing during the election about its concerns that Russia was trying to compromise some of his campaign aides. Instead, the bureau opened a counterintelligence investigation and secured a secret FISA warrant to spy on some of the campaign’s activities, including former adviser Carter Page.

The fact that the Clinton campaign was given the sort of defensive briefing Trump never received is certain to set off new alarm bells.

Sens. Charles Grassley, R-Iowa., and Ron Johnson, R-Wis., have been pressing recently to force more transparency into the question of defensive briefings for public officials being targeted by foreign powers.

“The Department of Homeland Security has made clear that ‘[a] secure and resilient electoral process is a vital national interest.’ An essential part of that process is ensuring that all candidates for office are treated fairly and are fully and equally prepared to address any potential security and counterintelligence concerns,” the senators wrote in a letter to the FBI. “The apparent absence of any policy, procedure, or practice for conducting defensive briefings undermines that process by risking the appearance of bias or, at worst, causing actual prejudice to a candidate for office.”

Expect defensive briefings to remain an issue well into the 2020 election.