The Pentagon is focusing on jailed Army Pfc. Bradley Manning as the main suspect in the leak of tens of thousands of secret U.S. military documents related to the war in Afghanistan, a senior Pentagon official told CNN.
Manning, 22, is believed to have accessed a worldwide military classified Internet and e-mail system to download tens of thousands of documents, according to the official, who would not be identified because of the ongoing criminal investigation of the soldier.
The official said investigators now believe Manning logged into a system called the Secret Internet Protocol Router Network, which essentially provides military members who have appropriate security clearances access to classified e-mails and the military's classified internet system.
But the official emphasized passwords and other control measures such as physical access are needed to log onto specific systems that provide information classified at the highest levels.
Pentagon officials have said for the past several days that so far the only material they have seen on WikiLeaks is classified at the "secret" level, a relatively low level designation that allows for a large number of military personnel to access the information.
The senior Pentagon official told CNN that for now, Defense Secretary Robert Gates is relying on the Army criminal investigation into Manning and the leaks to determine how it happened and what might need to be done to prevent future cases.
"The secretary is determined to get to the bottom of this," the official said.
The editor-in-chief of WikiLeaks, Julian Assange, has refused to say where his whistleblower website got about 91,000 United States documents about the war. The website published about 76,000 of them on Sunday in what has been called the biggest leak since the Pentagon Papers about the Vietnam War.
Manning was charged in June with eight violations of the U.S. Criminal Code for allegedly illegally transferring classified data, reportedly including an earlier video that wound up on WikiLeaks.org.
The Army is currently considering whether he should face the military equivalent of a trial over the charges.
He has not yet entered a plea, Army Maj. Bryan Woods told CNN, since there has not yet been a decision about whether he should face trial. Military lawyers for Manning referred CNN questions about him to Woods.
The Army has expanded its criminal investigation into Manning to look at potential accomplices and what military or U.S. government systems the information came from, Col. David Lapan at the Pentagon said Tuesday.
The military is "determined to find out who is responsible for this and to make sure they pay or are held accountable for it," Pentagon spokesman Geoff Morrell said on Monday.
"Look, there's a slew of people who have access. It will be, as all these investigations are, very difficult," Morrell said in an interview with CNN's John King. "This is a very big breach for which there must be consequences."
The investigation was expanded because investigators believe Manning has a connection to a number, but not all, of the 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 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." So far, he also has been charged with illegally adding unauthorized software to a classified computer system in connection to the leaking of the helicopter video.
Manning had top-secret clearance as an intelligence analyst with the Army while he was stationed in Iraq.
He was detained in June and sent to the U.S. base in Kuwait for his connection to the release of the classified U.S. military combat video showing the shooting deaths of Iraqi civilians and two journalists in 2007 by a helicopter gunship.
The Army is trying to trace who Manning's contacts were. They also want to know what computer servers he allegedly accessed in order to obtain a plethora of information from U.S. military documents to Department of State cables, according to the official.
Manning has not been cooperating with Army investigators; the official said Manning has invoked the Fifth Amendment and is refusing to answer questions from investigators.
Manning is still being held by the U.S. military in Kuwait while his unit begins to rotate back to the United States.
Manning could face additional charges as the Army's investigation continues, according to the official.
He has also been charged with communicating, transmitting and delivering national defense information to an unauthorized source and disclosing classified information concerning the national defense with reason to believe that the information could cause injury to the United States in connection to the leak of the same video.
CNN's Barbara Starr, Mike Mount and Richard Allen Greene contributed to this report