It was the literary leak heard around the world: a little-read detective novel called “The Cuckoo’s Calling,” by the debut author Robert Galbraith, was actually written by J. K. Rowling, one of the best-known authors in the world.

Now a law firm in London has sheepishly admitted that it was responsible.

Russells, a firm known for its work in the entertainment industry, said on Thursday that one of its partners, Chris Gossage, had told his wife’s best friend that the book, published in April, was actually written by Ms. Rowling.

That friend, Judith Callegari, revealed the information to a columnist for The Sunday Times of London in a Twitter exchange last week, then promptly deleted her tweets.

After doing its own investigation, The Sunday Times confronted Ms. Rowling with its findings, and she admitted to writing the book under a pseudonym. The newspaper published an article on Sunday identifying Ms. Rowling as the author.

But until Thursday, it remained a secret who was actually behind the leak of her identity.

In a statement released on Thursday, the law firm apologized “unreservedly” for the slip, explaining that Mr. Gossage had disclosed the information.

“Whilst accepting his own culpability, the disclosure was made in confidence to someone he trusted implicitly,” the statement said. “On becoming aware of the circumstances, we immediately notified J. K. Rowling’s agent. We can confirm that this leak was not part of any marketing plan and that neither J. K. Rowling, her agent nor publishers were in any way involved.”

When reached by telephone, John Reid, a partner at Russells, declined to comment further.

Through her publicist, Ms. Rowling released a statement saying that she was “disappointed.”

“A tiny number of people knew my pseudonym and it has not been pleasant to wonder for days how a woman whom I had never heard of prior to Sunday night could have found out something that many of my oldest friends did not know,” she said. “I had assumed that I could expect total confidentiality from Russells, a reputable professional firm and I feel very angry that my trust turned out to be misplaced.”

Article source: http://www.nytimes.com/2013/07/19/business/media/another-rowling-mystery-solved-behind-the-tweet-that-identified-her.html?partner=rss&emc=rss