Unauthorised memoir on the raid that killed Osama bin Laden said to be a threat to military families after Pentagon review

The much-anticipated insider account of the Navy Seal raid that killed Osama bin Laden hit bookstores in the US on Tuesday, having already knocked this year's publishing leviathan Fifty Shades of Grey off its bestseller perch.

Sales of the unauthorised memoir, No Easy Day, went ahead despite a Pentagon threat of legal action against its author for alleged violation of non-disclosure agreements.

Before the Pentagon's warning last week, publisher Dutton brought the release date forward from 11 September, saying it was important to let the book speak for itself.

On Tuesday the Pentagon said that after reviewing the book, it believed that it contains classified information. Rear Admiral Sean Pybus, who heads the Naval Special Warfare Command, said it could also provide enemies of the US with insight into their operations.

He told his force in a letter quoted by the Associated Press that "hawking details about a Seal mission" and selling other details of Seal training and operations puts the force and their families at risk.

"For an elite force that should be humble and disciplined for life, we are certainly not appearing to be so," Pybus wrote in a letter to the 8,000 troops under his command. "We owe our chain of command much better than this."

At the Pentagon, press secretary George Little said an official review of the book determined that it reveals what he called "sensitive and classified" information, but he would not give details about which passages in the book were considered to be a violation of the non-disclosure agreements signed by Mark Bissonette, the real name of the Navy Seal who wrote the account under the pseudonym Mark Owen.

Fox News identified Bissonette, 36, as the author of the memoir after Penguin announced the book last month and his identity was confirmed by military sources.

In a letter addressed to "Mark Owen", Charles Johnson, the defence department general counsel, alleged the writer violated secrecy agreements and broke federal law. "In the judgment of the Department of Defense, you are in material breach and violation of the nondisclosure agreements you signed. Further public dissemination of your book will aggravate your breach and violation of your agreements," Johnson wrote.

Bissonnette's lawyer, Robert Luskin, disputed this, saying he believes the decorated former Seal has "earned the right to tell his story."

Luskin's letter in response said the author "sought legal advice about his responsibilities before agreeing to publish his book and scrupulously reviewed the work to ensure that it did not disclose any material that would breach his agreements or put his former comrades at risk".

The controversy does not appear to have hurt sales of the memoir. Presales of the book at Amazon overtook the record-breaking erotic trilogy Fifty Shades of Grey, while an initial print run of 200,000 has been increased to 575,000 copies, according to Publishers Weekly.

The book reveals details of the raid that differ form the official version of events and raises questions as to whether Bin Laden could have been taken alive .

According to the author, Bin Laden was shot not at point-blank range in a bedroom but from a distance in a hallway as he peered around his bedroom door. He was unarmed and it was later discovered that weapons around the bed were not loaded, which Bissonette cites as being a mark of dishonour. "There is no honor in sending people to die for something you won't even fight for yourself," he writes in the book.

Bissonnette said he was behind the "point man," or lead commando, as the Seals followed Bin Laden into the room. They discovered him on the floor at the foot of his bed with "blood and brains spilled out of the side of his skull", and two women wailing over "still twitching and convulsing" body.

The official version of events is that the lead commando missed and the Seals confronted Bin Laden in his bedroom in the May 2011 raid, killing him with one shot to the chest and another in the left eye.

Military officials have said that the Seals made split-second decisions, fearing that he could have been wearing a suicide vest, but critics have argued that despite being labelled a "kill or capture" mission, there was virtually no chance he would be brought back alive.