The source of the photo, which appears to show a special operations soldier in leveling an automatic rifle during a training exercise, was not immediately clear.

“Here is the first picture of the dog who murdered the martyr Shaykh Usama Bin Laden,” wrote one of the posters, using an alternate spelling of bin Laden’s name. “May Allah have mercy on him.”

Fox News on Thursday identified the author of the book, which is titled “No Easy Day: The Firsthand Account of the Mission That Killed Osama bin Laden,” as a 36-year-old former SEAL from Wrangell, Alaska. The Associated Press later said it had confirmed the author’s identity. (NBC News is not identifying the former SEAL.)

Penguin Group (USA)’s Dutton imprint, the publisher, asked news organizations Thursday to withhold his identity.