Multiple media sources have now confirmed that American academic Stefan Halper is the “top secret” informant the FBI asked to sidle up to Trump campaign officials in 2016. Some questions follow: Who asked Mr. Halper to keep tabs on the Trump officials, and when and why?

The answers go to the credibility of the Federal Bureau of Investigation’s claim that it didn’t open an official counterintelligence probe into Trump-Russia collusion until July 31, 2016. The answers might also show if Obama Administration officials knew about...