A court case reveals the dance between reporter and source

“I AM ASKING YOU AGAIN TO TRUST ME,” the reporter wrote his source. “I am very good at my job.”

And, he added: “Get ready for prime time. The reckoning is beginning…. You and everyone else will be protected.”

That reporter, BuzzFeed’s Jason Leopold, was right about a few things. He is good at his job. A reckoning was beginning. But he was wrong about this: His source—Natalie Mayflower Sours Edwards—would not be protected.

After leaking thousands of documents to Leopold from her post at the Treasury Department’s Financial...