“Issuing subpoenas for documents means very little,” Jeffress said. “It just means they’ve concluded they need the law to get documents. When they start issuing subpoenas for testimony, it means they’re serious.”

Watch for subpoena battles. When someone receives a subpoena, they can fight it if they don’t want to comply, Jones said.

Why wouldn’t someone comply? Maybe they think the request violates their Fourth Amendment right to privacy or that it’s beyond the scope of Mueller’s inquiry. Fighting the subpoena would involve filing a motion with a district court judge in D.C. Those motions would likely be redacted, but again, leakers may point out when such a fight is ongoing.

What about immunity? Pay attention if there’s news a witness was granted immunity.