Washington (CNN) -- A Pentagon official said Tuesday that a former Navy SEAL who helped kill Osama bin Laden included classified material in his new book and did not follow protocol for pre-publication review.
Also, on the same day the much-anticipated memoir hit book shelves, CNN obtained a copy of message written by the SEALs' commander to members of his unit.
In it, Rear Adm. Sean Pybus, head of the Naval Special Warfare Command, said he was "disappointed, embarrassed and concerned" that troops are now openly speaking and writing about their secret work. "We do NOT advertise the nature of our work, NOR do we seek recognition for our actions." The emphasis is the admiral's.
Pre-orders put "No Easy Day: The Firsthand Account of the Mission That Killed Osama Bin Laden" at No. 1 on Amazon's bestseller list for two weeks.
But the Pentagon was not as as eager to see the release of the book.
Pentagon press secretary George Little told reporters that officials believe the book contains sensitive and classified material.
The Pentagon's determination, to a large extent, centers around several instances where author Matt Bissonnette, writing under the pen name "Mark Owen," describes how SEAL units are organized, trained and operate, said a U.S. official, rather than solely on specifics of the bin Laden mission, which are largely known.
Describing the instances in a very broad sense, the official said, "does this compromise national security today, probably not, but do we talk about this stuff, no."
The official declined to be identified because of the sensitive nature of the information.
Last week, the Pentagon's general counsel threatened legal action against the man who wrote the revealing book.
In a letter addressed to "Mark Owen," General Counsel Jeh Charles Johnson 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 responded in a letter last week saying 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."
Officials are concerned about protecting classified information, and not preventing the telling of a story, according to Little. The Defense Department is reviewing what options it may have, according to Little.
"When you have special operations units that perform these missions, there are tactics, techniques, and procedures, not to mention human life, that are in play," Little said. "And it is the height of irresponsibility not to have this kind of material checked for the possible disclosure of classified information."
Pre-release of copies of the book and the Pentagon's lack of sufficient time to act made it impractical to ask the publisher to not release the book, Little said.
The Pentagon has not banned copies of the book from military exchanges.
Publisher Dutton had originally planned to release 300,000 copies, but after fevered hype, it increased the number to 575,000, according to Publisher's Weekly.
In the 336-page book, Bissonnette wrote about his concerns that bin Laden would put up a fight, after the SEALs' helicopter crashed near bin Laden's compound.
"Roughly 15 minutes had passed and bin Laden had plenty of time to strap on a suicide vest or simply get his gun," he says in the book.
Bin Laden was not wearing a vest when he died. His two guns, according to Bissonnette, were on a shelf in his bedroom, apparently untouched.
After the helicopter crash and a hard fight to get through the compound's defenses, Bissonnette and several other SEALS were near the top floor of the compound, where intelligence predicted bin Laden would be.
"We were less than five steps from getting to the top when I heard suppressed shots. BOP. BOP.
"The point man had seen a man peeking out of the door on the right side of the hallway about 10 feet in front of him. I couldn't tell from my position if the rounds hit the target or not. The man disappeared into the dark room."
The man who peeked out the door had been shot, but was still moving when the SEALs entered the room. Bissonnette described the end.
"In his death throes, he was still twitching and convulsing. Another assaulter and I trained our lasers on his chest and fired several rounds. The bullets tore into him, slamming his body into the floor until he was motionless."
But there was still the question of whom they had killed. One of Bissonnette's roles was photographing the body.
"It was strange to see such an infamous face up close. Lying in front of me was the reason we had been fighting for the last decade. It was surreal trying to clean blood off the most wanted man in the world so that I could shoot his photo. I had to focus on the mission, right now we needed some good quality photos."
Bissonnette said he had mixed feelings about President Barack Obama, who ordered the operation.
"None of us were huge fans of Obama. We respected him as the commander-in-chief of the military and for giving us the green light on the mission," wrote the former SEAL, who was a 36-year-old chief petty officer when he left the Navy as a highly decorated commando in April.
CNN's Ashley Fantz and Lateef Mungin contributed to this report.