The US state department has told the BBC it believes an alleged whistle-blower obtained secret diplomatic data despite being at a field base in Iraq.
Serviceman Bradley Manning, 22, faces two charges related to the illegal transfer and transmission of classified information from a US military network.
The US said he was suspected of downloading from SIPR Net.
He reportedly then passed on the data, including army videos and diplomatic messages, to the WikiLeaks website.
WikiLeaks has repeatedly said it does not have the confidential messages and the site itself is not mentioned in the charges against Private First Class (Pfc) Manning.
A former hacker, Adrian Lamo, reported Pfc Manning to the US authorities. He said the intelligence analyst admitted, in a series of online chats, to sending data to the whistle-blowing website.
In the redacted charge sheet detailing the accusations against Pfc Manning, the Army alleges that he transmitted, "to a person not authorised to receive it", a classified US Department of State cable described as "Reykjavik 13".
The US also alleges Pfc Manning obtained 150,000 US diplomatic cables without proper authorisation.
In February this year, WikiLeaks released a diplomatic cable from 13 January 2010 recording details of a meeting in Iceland between US diplomat Sam Watson, British ambassador Ian Whitting, and members of the Icelandic government.
Now the state department has told BBC News how Bradley Manning, based at the Hammer military field base in Iraq, could have accessed information unrelated to the US mission in that country.
In an e-mail, US state department spokesperson Megan Mattson said: "After the events of 11 September 2001, agencies across the federal government understood that greater information sharing was vital to protecting our national security interests.
"As part of our efforts to make Department of State information available to those who have a legitimate need to know, we established the Net Centric Diplomacy initiative, which allows Department of State information to be shared on the Department of Defence's SIPR (Secret Internet Protocol System) Net system."
Ms Mattson said that access to the system was only permitted to those "civilian and military users with appropriate security clearances".
She said that Bradley Manning was "suspected of violating the trust and confidence given to him".
Catherine Lotrionte, associate director of the Institute for Law, Science and Global Security, has a background in US intelligence work
She told BBC News that there was "a push after 9/11 that information was going to be shared - and databases connected."
In her view, data-sharing is necessary for effective intelligence work, and the risk that it may make large data breaches easier is simply "the cost of doing business - the downside is that someone may break the rules".
Speaking on condition of anonymity, a retired US General, with extensive military intelligence experience, told BBC News that there were, "layers of clearances designed to protect and restrict access to data."
He said that sharing information was the right thing to do and the military benefits far outweighed the risks.
But Crispin Black, a former intelligence analyst for the UK government, says the content of cables can be very sensitive.
"Diplomatic cables don't usually contain huge secrets but they do contain the unvarnished truth so in a sense they can be even more embarrassing than secrets."
He told the BBC that the possibility that someone in a base in Iraq could potentially access cables about Iceland violated, the principle of "need to know" in intelligence.
According to claims by Adrian Lamo, Pfc Manning told him in online chats that he removed information by burning it onto a CD.
Mr Lamo claims that Pfc Manning told him that he disguised his activities by pretending he was listening to music by Lady Gaga.
According to Lamo, Pfc Manning is alleged to have said in one online-chat that "Hilary Clinton, and several thousand diplomats around the world are going to have a heart attack…"
WikiLeaks has consistently denied possessing the thousands of diplomatic cables Mr Lamo alleges were passed to them.
WikiLeaks editor Julian Assange told BBC News on Thursday that he was "disturbed" by the charges against Bradley Manning.
Mr Assange said that it was "clear that some of those charges relate to information that should not have been classified".
While WikiLeaks says that it is technically impossible for it to know if Pfc Manning is indeed its source, it is trying to assist in his defence.
Mr Assange said that contact had not been established with Pfc Manning personally but that he expected that would change shortly.
He said that whoever had leaked the information was a "hero" for exposing wrong-doing by the US military and accused the army of a "double standard" in prosecuting Pfc Manning.