U.S. intelligence operatives covertly sabotaged a prominent al-Qaeda online magazine last month in an apparent attempt to sow confusion among the group’s followers, according to officials.

The operation succeeded, at least temporarily, in thwarting publication of the latest issue of Inspire, the English-language magazine distributed by al-Qaeda in the Arabian Peninsula. When it appeared online, the text on the second page was garbled and the following 20 pages were blank. The sabotaged version was quickly removed from the online forum that hosted it, said independent analysts who track Islamist militant Web sites.

It is unclear how the hacking occurred, although U.S. intelligence agencies, including the National Security Agency and the CIA, have invested heavily in cyber-capabilities in recent years. Security officials, speaking on the condition of anonymity, said the recent operation was only the latest U.S. attempt to disrupt al-Qaeda’s online propaganda.

“You can make it hard for them to distribute it, or you can mess with the content. And you can mess with the content in a way that is obvious or in ways that are not obvious,” said one intelligence official, who, like others, spoke on the condition of anonymity in order to discuss sensitive internal debates.

Officials at the Office of the Director of National Intelligence, which oversees the government’s 16 intelligence agencies, declined to comment, as did the White House and the Pentagon.

The hacked version of Inspire magazine appeared May 14, said Evan Kohlmann, an analyst who tracks jihadist Web sites. His firm, Flashpoint Global Partners, captured an image of the issue, which featured a cover showing a fighter in a heavy coat, shouldering a rocket-propelled grenade launcher and a Kalashnikov rifle. The title was “How Did It Come to This?”

Within half an hour of its appearance, the magazine was removed, presumably in response to the hacking, Kohlmann said.

On May 30, a new version, Issue 11, appeared. That issue portrayed the Boston Marathon bombing as vindication of Inspire’s message that “a single lone jihad operation can force America to stand on one foot and live in a terrified state, full of fear.”

Inspire comprises first-person accounts of operations, exhortations to jihad and do-it-yourself advice for extremists. A second intelligence official said the publication is seen as a threat because it “has a specific readership — a following. People will look for it, as opposed to something randomly posted. Two, it is very user-friendly. Inspire uses pictures and step-by-step diagrams, and that’s a problem.”

Does disruption work?

The decision to disrupt the magazine last month was part of a debate within the Obama administration over the response to online publications that promote radicalization.

The debate spiked after the April 15 Boston Marathon bombing. One of the suspects, Dzhokhar Tsarnaev, 19, told the FBI that he and his late brother, Tamerlan Tsarnaev, 26, learned from the magazine how to make the pressure-cooker bombs used in the attack. He also told them they had been inspired by sermons and other material from the Internet, said officials briefed on the disclosures.

“There’s a robust debate in the community about where do you draw the line on whether or not you should interfere with or take down certain sites,” the second intelligence official said.

Current and former government officials said the debate has been swayed by an argument that Inspire represents an incitement to imminent lawless action, which outweighs First Amendment protections. A 2011 Justice Department “white paper” invoked a similar concept in debates over the lethal targeting of U.S. citizens.

Incitement to violent action is an Inspire staple. A 2010 issue, for example, provided instructions for turning a pickup truck into “The Ultimate Mowing Machine” by welding steel blades onto the front at headlight level — about the height of a human torso — and then plowing into a crowd “to strike as many people as possible.”

The FBI later investigated a homegrown terrorist cell on the East Coast that discussed using the mowing-machine technique, said a consultant who worked on the investigation.

“I don’t think al-Qaeda has a First Amendment right to put out its propaganda, to encourage people to commit acts of terrorism,” said Rep. Adam B. Schiff (D-Calif.), a member of the House Intelligence Committee, who declined to comment on specific cases. “Unfortunately, I think Inspire magazine is a significant threat to the extent that it disseminates information about how to build a bomb or encourages people to get radicalized. It has shown a dangerous effectiveness. And one that’s difficult to address.”

Others contend that disruption is not the best long-run strategy. “The only way that you’re really going to be effective is to help amplify more mainstream moderate Muslim voices,” said Michael E. Leiter, former director of the National Counterterrorism Center. “That’s vastly more effective than trying to disrupt radical voices.”

In the case of Inspire, the debate stretches back three years. The first issue contained a recipe for making a bomb using common materials, such as nails and a pressure cooker like the ones used in Boston. The title of the article was “Make a Bomb in the Kitchen of Your Mom.”

There was also a threat to Molly Norris, a Seattle cartoonist who published a satirical cartoon about the prophet Muhammad. “She should be taken as a prime target of assassination,” wrote Anwar al-Awlaki, the American-born cleric who was later killed in a U.S. drone strike.

“It’s obvious if people are calling for crazies to murder a U.S. citizen, why wouldn’t you stop it?” said one former official, recalling the debate in which Gen. Keith B. Alexander, director of the National Security Agency, argued on behalf of disruption.

In that case, the administration decided against action, in part because the CIA preferred to use the site to gather intelligence. In subsequent debates, the danger of an imminent threat “really made the difference” in terms of whether to disrupt issues of the magazine, according to a former administration official.

Attacks on production

Although techniques are carefully guarded, officials said U.S. intelligence operatives have monitored the magazine during its production process through overseas computer networks. Each time an issue is about to hit the Internet, officials from the NSA, the CIA, the Pentagon, the State Department and the Justice Department debate whether to sabotage it.

In cases where threats appear imminent, steps might be taken to disrupt publication. In some cases, cyberspies sabotage files so that they come up blank when a user clicks on them, according to the former official. In one case, the official said, the sabotage was not corrected for months.

Sometimes, the disruption occurs when the magazine is being put together, intelligence officials said. An U.S. operator might alter a technical point in a set of bomb-making instructions so the device will not work. The sabotage could go unnoticed for a long time, an official said.

Still, the disruptions are temporary, and the content usually makes its way online.

The quality of Inspire’s writing diminished after Awlaki and Samir Khan, the American-born editor of the magazine, were killed in a drone strike in Yemen in September 2011. Despite intelligence assessments that Inspire might disappear after the deaths, it has continued to publish.

Karen DeYoung contributed to this report.