Anxious that Wikileaks may be on the verge of publishing a batch of secret State Department cables, investigators are desperately searching for founder Julian Assange. Philip Shenon reports. Plus, Daniel Ellsberg tells The Daily Beast: "Assange is in Some Danger."

(This story has been updated to reflect new developments on Assange's whereabouts, including the cancelation of a scheduled appearance in Las Vegas.)

Pentagon investigators are trying to determine the whereabouts of the Australian-born founder of the secretive website Wikileaks for fear that he may be about to publish a huge cache of classified State Department cables that, if made public, could do serious damage to national security, government officials tell The Daily Beast.

The officials acknowledge that even if they found the website founder, Julian Assange, it is not clear what they could do to block publication of the cables on Wikileaks, which is nominally based on a server in Sweden and bills itself as a champion of whistleblowers.

“We’d like to know where he is; we’d like his cooperation in this,” one U.S. official said of Assange.

American officials said Pentagon investigators are convinced that Assange is in possession of at least some classified State Department cables leaked by a 22-year-old Army intelligence specialist, Bradley Manning of Potomac, Maryland, who is now in custody in Kuwait.

And given the contents of the cables, the feds have good reason to be concerned.

As The Daily Beast reported June 8, Manning, while posted in Iraq, apparently had special access to cables prepared by diplomats and State Department officials throughout the Middle East, regarding the workings of Arab governments and their leaders, according to an American diplomat.

The cables, which date back over several years, went out over interagency computer networks available to the Army and contained information related to American diplomatic and intelligence efforts in the war zones in Afghanistan and Iraq, the diplomat said.

American officials would not discuss the methods being used to find Assange, nor would they say if they had information to suggest where he is now. "We'd like to know where he is; we'd like his cooperation in this," one U.S. official said of Assange.

Assange, who first gained notoriety as a computer hacker, is as secretive as his website and has no permanent home.

He was scheduled to speak Friday in Las Vegas at an Investigative Reporters and Editors conference. But the group’s executive director, Mark Horvit, tells The Daily Beast that Assange canceled the appearance—he was on a panel to discuss anonymous sources—within the last several days as a result of unspecificed “security concerns.” Horvit said he communicated with Assange through email and did not know where he might be.

Last week, Assange was scheduled to join famed Pentagon Papers leaker Daniel Ellsberg for a talk at New York's Personal Democracy Forum. Assange appeared via Skype from Australia instead, saying lawyers recommended he not return to the United States.

Julian Assange, in April 2010, discussing confidential sources in the digital age

Assange was in the United States as recently as several weeks ago, when he gave press interviews to promote the website’s release of an explosive 2007 video of an American helicopter attack in Baghdad that left 12 people dead, including two employees of the news agency Reuters.

Wikileaks has not replied directly to email messages from The Daily Beast.

However, in cryptic messages he sent this week via Twitter, Wikileaks referred to an earlier Daily Beast article on the investigation of Manning and said that it “looks like we’re about to be attacked by everything the U.S. has.”

In an earlier post, the site said that allegations that “we have been sent 260,000 classified U.S. embassy cables are, as far as we can tell, incorrect.”

This morning, a new Wikileaks tweet went out: "Any signs of unacceptable behavior by the Pentagon or its agents towards this press will be viewed dimly."

Pentagon investigators say that particular post may have been an effort by Wikileaks to throw them—and news organizations—off the track as the site prepared the library of State Department cables for release, officials said.

“It looks like they’re playing some sort of semantic games,” one American official said of Wikileaks. “They may not have 260,000 cables, but they’ve probably got enough cables to make trouble.”

• Philip Shenon: The State Dept.’s Worst Nightmare• Daniel Ellsberg: 'Assange Is In Danger' In another cryptic Twitter message, the site said that while the State Department might be alarmed about the prospect of the release of classified cables, “we have not been contacted.”

American officials were unwilling to say what would happen if Assange is tracked down, although they suggested they would have many more legal options available to them if he were still somewhere in the United States.

Manning has reportedly admitted that he downloaded 260,000 diplomatic cables and provided them to Wikileaks. In Internet chat logs first revealed by Wired magazine, Manning also took credit for leaking the 2007 video to the website.

“Hillary Clinton and several thousand diplomats around the world are going to have a heart attack when they wake up one morning and find an entire repository of classified foreign policy is available,” Manning wrote of the diplomatic cables, according to Wired.

Wikileaks has not confirmed that Manning is a source of any information posted on the site. “We do not know if Mr. Manning is our source, but the U.S. military is claiming he is, so we will defend him,” Wikileaks said in another Twitter message.

Manning was turned in to the Pentagon by a former computer hacker based in California, Adrian Lamo, after Manning approached Lamo for counsel. Manning is believed to have contacted Lamo after reading a recent profile of him in Wired.

In the chat log revealed by Wired, Manning bragged to Lamo about having downloaded a huge library of State Department cables, as well as the 2007 video of the helicopter attack, and having provided the material to Wikileaks.

Manning took credit for having leaked a classified diplomatic cable that has already appeared on the site—a memo prepared by the United States embassy in Reykjavik, Iceland, that described a meeting there between American and Icelandic officials over that country’s banking meltdown.

The January 2010 memo may have been of special interest to Wikileaks given the site’s close ties to Iceland, where Assange has based himself at times and where he worked with local lawmakers to draft free-speech laws that give broad freedom to journalists to protect their sources.

A profile this week in The New Yorker magazine depicted Assange feverishly at work with Icelandic colleagues in Reykjavik in March as he organized the release of the 2007 video of the helicopter attack. The edited video was given the title Collateral Murder, and its release infuriated officials at the Defense Department.

With its network of whistleblowers, Wikileaks has published documents and videos on its site that have outraged other foreign governments. To protect the site from attack by intelligence agencies, Assange has placed Wikileaks on several Internet servers, making it all but impossible for any government to shut down the site entirely.

Philip Shenon, a former investigative reporter at The New York Times, is the author of The Commission: The Uncensored History of the 9/11 Investigation.