LONDON — Ian Cobain, a reporter with The Guardian, is one of very few people who know why a student arrested by armed British police officers in 2013 was finally acquitted this year of terrorism charges.

Problem is, he cannot report what he knows. He was allowed to observe much of the trial, but only under strict conditions intended to keep classified material secret. His notebooks are being held by Britain’s domestic intelligence agency. And if he writes — or even talks — about the reason that the student, Erol Incedal, 27, was acquitted, Mr. Cobain faces prosecution and possibly jail.

“I know the essence of what was happening,” Mr. Cobain said, “but I can’t tell, I can’t even talk to my editor about this.”

Having initially gone along reluctantly with the reporting restrictions, a number of British news organizations are now challenging them in court. And yes, the challenge itself is being heard under secrecy rules that leave the public mostly excluded. Were Mr. Cobain to break the law and disclose what he knows publicly, his prosecution would also take place in secret.