(CNN) -- U.S. officials from the president down tried Tuesday to downplay the leak of tens of thousands of documents about the war in Afghanistan, a disclosure experts are calling the biggest leak since the Pentagon Papers about Vietnam.
Pentagon officials have not found anything top-secret among the documents, a Defense Department spokesman said.
"From what we have seen so far, the documents are at the 'secret' level," Col. David Lapan said. That's not a very high level of classification.
Lapan emphasized that the Pentagon has not looked at all of the more than 75,000 documents published on WikiLeaks.org on Sunday.
President Barack Obama said Tuesday that he is "concerned about the disclosure of sensitive information" about the U.S. mission in Afghanistan but asserted that the documents don't shed much new light on the issue.
Senate Foreign Relations Committee Chairman John Kerry, D-Massachusetts, said Tuesday that the importance of the leak should not be overstated.
"I think it's important not to overhype or get excessively excited about the meaning of those documents," Kerry told the committee.
But, he said, the leak "breaks the law, and equally importantly, it compromises the efforts of our troops, potentially, in the field and has the potential of putting people in harm's way," he said.
The top-ranking U.S. military officer, Adm. Mike Mullen, said he was "appalled" by the leak but questioned the current significance of the documents, which date from 2004 to 2009.
"Much has changed since 2009, particularly with respect to our focus, our new strategy in Afghanistan and Pakistan," said Mullen, the chairman of the Joint Chiefs of Staff, in Baghdad, Iraq. "A lot of it is focused on the past, and I am very focused on the future."
Afghan President Hamid Karzai has ordered the Foreign Ministry and National Security Council to study the vast cache of documents, Karzai's office said Tuesday.
Meanwhile, the U.S. Army has expanded its criminal investigation into a soldier allegedly involved in the earlier leak of a combat video and thousands of military documents, according to Col. Lapan.
According to a U.S. military official who asked not to be named because he was not authorized to talk about the investigation, the probe by the Army's Criminal Investigation Division into Pfc. Bradley E. Manning was expanded to look at potential accomplices and what military or U.S. government systems the information came from.
CNN was unable to reach Manning's attorney.
The leak of documents published by the website WikiLeaks.org is a "very big breach," Pentagon spokesman Geoff Morrell said on Monday.
The investigation was expanded because investigators believe Manning has a connection to a number, but not all, of documents released on WikiLeaks.org on Sunday. The Army is also working with other U.S. agencies in the investigation, according to Army CID spokesman Christopher Grey.
Manning, 22, of Potomac, Maryland, earlier was charged with eight violations of the U.S. Criminal Code for allegedly illegally transferring classified data, including a video that wound up on WikiLeaks. He has been accused of "wrongfully introducing a classified video of a military operation filmed at or near Baghdad, Iraq" around July 12, 2007, "onto his personal computer, a non-secure information system."
The documents released by WikiLeaks Sunday are divided into more than 100 categories. Tens of thousands of pages of reports document attacks on U.S. troops and their responses, relations between Americans in the field and their Afghan allies, intramural squabbles among Afghan civilians and security forces, and concerns about neighboring Pakistan's ties to the Taliban.
And WikiLeaks has another 15,000 documents that it plans to publish after editing out names to protect people, according to the website's founder and editor in chief, Julian Assange.
He said on CNN's "Larry King Live" that the first-hand accounts represent "the cut and thrust of the entire war over the past six years," through the military's own raw data: numbers of casualties, threat reports and notes from meetings between Afghan leaders and U.S. commanders.
"We see the who, the where, the what, the when and the how of each one of these attacks," Assange said. That includes, he said, possible evidence of war crimes by both U.S. troops and the Taliban, the Islamic militia that has been battling U.S. troops since 2001.
Assange said some events listed in the reports are "very suspicious," such as reports of skirmishes in which "a lot of people are killed, but no people taken prisoner and no people left wounded."
"In the end, it will take a court to really look at the full range of evidence to decide if a crime has occurred," he said. But earlier, he noted, "This material does not leave anyone smelling like roses, especially the Taliban."
One leaked report shows that truckers moving vital supplies along the roads of war-torn Afghanistan have faced shakedowns by both the Taliban and Afghan authorities, with Taliban fighters charging up to $500 for safe passage.
A trucking company working in Afghanistan told American forces "that they were approached by Taliban personnel to talk about payment for the safe passage of convoys through their area," one 2007 report states. "The current price for passage is $500 US per truck from Kandahar to Herat, $50 US per truck from Kabul to Ghazni, $100 US per truck from Ghazni to Orgun-E, and $200-300 US per truck from Orgun-E to Wazi Kwah."
CNN has not independently confirmed the authenticity of the documents, but neither the White House nor the Pentagon has denied that they are what WikiLeaks claims they are.
"I don't think that what is being reported hasn't in many ways been publicly discussed -- whether by you or by representatives of the U.S. government -- for quite some time," White House spokesman Robert Gibbs told reporters. But he said an investigation into the source of the leak had begun by last week.
"There is no doubt that this is a concerning development in operational security," he said.
Assange said WikiLeaks withheld some documents that dealt with activity by U.S. Special Forces and the CIA, "and most of the activity of other non-U.S. groups."
But he said the documents reveal the "squalor" of war, uncovering how a number of small incidents have added up to huge numbers of civilian deaths.
"What we haven't seen previously is all those individual deaths," he said. "We've seen just the number. And like Stalin said, 'One man's death is a tragedy; a million dead is a statistic.' So, we've seen the statistic."
The release of the documents is being called the biggest intelligence leak in history, drawing comparisons to the disclosure of the Vietnam-era Pentagon Papers.
"There hasn't been an unauthorized disclosure of this magnitude in 39 years," said Daniel Ellsberg, the onetime Pentagon official who leaked that multiple-volume secret history of the conflict.
Others disagreed with the comparison. Bruce Riedel, an analyst at the Saban Center for Middle East Policy at the Brookings Institution, noted that the Pentagon Papers were part of a document prepared for U.S. leaders that analyzed how the United States got into Vietnam, "which assessed successes and failures in a comprehensive way."
"This is really the raw material of the war -- unassessed, raw, fragmentary data that I think in each case, you have to be very careful how much of a larger picture you can conclude from these fragments and snippets," Riedel said.
But Ellsberg said the documents, "low-level as they are," raise the question of whether the United States has a winning strategy in Afghanistan and whether it should continue to pursue the war.
"They do give us the sense of the pattern of failure, of stalemate, and why we're stalemated -- civilian casualties that recruit for the Taliban ... and raise the question of what we're doing there," he said.
The United States and its allies invaded Afghanistan in 2001 after the September 11 attacks on New York and Washington. The attacks were carried out by the Islamic terrorist network al Qaeda, which operated from bases in Afghanistan with the approval of the Taliban, the fundamentalist movement that ruled most of the country at the time.
The leaked documents provide fleeting glimpses into the possible whereabouts of Osama bin Laden in the years since his escape from American forces at Tora Bora in the Afghan mountains.
Documents published in the British newspaper The Guardian quoted intelligence sources as saying bin Laden wanted al Qaeda operatives disguised as journalists to attack Afghan President Hamid Karzai during a news conference in 2004. In 2005, his financial adviser and an Afghan insurgent leader reportedly were dispatched to obtain rockets from North Korea to use against U.S. forces in Afghanistan.
In 2006, he was reported to be attending monthly meetings in the Pakistani city of Quetta with fellow fugitives from the leadership of the Taliban, the Islamic militia that hosted al Qaeda when it controlled most of Afghanistan. Another report the same year states that he arranged a marriage for a valued lieutenant, a specialist in building roadside bombs.
Neither the documents cited only by the Guardian, which had advance access to the Wikileaks documents released Sunday, nor the information contained in them could be independently verified by CNN. The CIA would not comment on secret documents. But a U.S. counterterrorism official said on condition of anonymity that American authorities believe that the al Qaeda leader "has gone into deep hiding."
"We think he's spending a heck of a lot of time trying to avoid being captured or killed," the official said. "After all, he's seen many of his top lieutenants taken off the battlefield since 9/11 - and especially over the past two years. We haven't had a firm fix on his location for a number of years. If we did, he wouldn't be there. The aggressive search for him continues without pause."
Assange, meanwhile, said his website is not campaigning against the war.
"WikiLeaks does not have an opinion whether the war in Afghanistan should continue or not continue. ... It should continue in a just way if its to continue at all," he said.
CNN Pentagon Correspondent Barbara Starr and CNN's Atika Shubert, Richard Allen Greene, David DeSola, Adam S. Levine, Mike Mount and Atia Abawi contributed to this report.