Among Mr. Klokov’s alleged crimes, according to investigators, were saying Russia was evacuating wounded troops to Belarus and hiding the true death toll among soldiers; that Ukraine isn’t run by Nazis; and that Russian soldiers were killing Ukrainian civilians.

The morning after Mr. Klokov’s arrest on March 16, he told investigators that he misspoke about the military operation and could have been misled by information he had received, according to a transcript prepared by investigators. During a second conversation, he confessed.

“I made a mistake while in an emotional and anxious state,” he told police, according to the transcript.

The case files don’t indicate whether authorities had a court order to tap Mr. Klokov’s phone. Mr. Klokov’s lawyer, Mr. Berman, said an investigator told him in early April that they had procured a court order, but he said he hasn’t seen it.