Teixeira, 21, is said to have overseen the private online group, containing around 25 active members, in which the leaked documents were shared.

He is a member of the intelligence wing of the Massachusetts Air National Guard, according to interviews and documents reviewed by The New York Times.

Officials told Sky News' US partner network, NBC News, that they have been on to him for some time and that an arrest is imminent.

The FBI says it has made an arrest and is continuing law enforcement activity in Massachusetts.

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It comes after US President Joe Biden said he was "concerned" about the widespread leaks, that have rocked The Pentagon.

While their accuracy has been questioned, the leaked documents appear to reveal potential vulnerabilities in Ukraine's defences, as well as the numbers of UK special forces allegedly active in the country.

Mr Biden said that while he was "concerned" sensitive information had been leaked, "there's nothing contemporaneous that I'm aware of that is of great consequence".

He noted there was a "full-blown" investigation by the intelligence community and the US Justice Department. "We're getting close," he said, on the probe into what happened.

"But I don't have an answer."

Known as "OG" in the chatroom, the suspect behind the leaks allegedly claimed to spend parts of his day inside a secure facility where phones were banned, according to a story published by the Washington Post.

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What do the highly classified documents say?

He initially typed up versions of highly secret documents and posted them for the group of about 25 active members from various countries to read, but later switched to posting slews of photos of the documents themselves.

As well as sharing state secrets, OG would also forecast major events before they became headline news, according to another member of the chat interviewed by the Post.

However, they told the Post that OG was "not interested in helping any foreign agency" and that he was a "young charismatic man" who was attempting to impress friends.

How were the documents leaked?

The Pentagon is racing to find the culprit and the scale of the scandal has been compared to the Edward Snowden leaks in 2013.

Various theories have emerged about how the highly classified documents came to be made public, including a potential Russian hack.

But defence and security expert Professor Michael Clarke said that the leak could be an issue with the number of people who have access to classified documents in the US.

Unlike the UK, which has a "need to know" policy in relation to intelligence information, in the US they have a "dare to share" philosophy which means an estimated 1.3 million people have access to secret and top secret documents.

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Speaking to the Sky News Daily Podcast, he said: "In the American system, it is 'dare to share' which is to get everybody on board with a common picture of what we're talking about and, of course, it's based on the idea that proper Americans are patriotic enough not to leak material."

"But if 1.3 million people have got access to some form of secret material then even a small proportion - 0.1% of those people - if they are geeks who want to be a bigshot amongst their friends, as this person probably did…then some people like that will emerge from the system."

What do the documents show?

As more and more of the information contained in the documents is reported, the reach of American intelligence agencies is increasingly under the spotlight.

According to the New York Times, the documents also illuminate the depth of infighting inside the Russian government.

They paint a picture of feuding behind the walls of the Kremlin over the number of casualties in the Ukraine war.

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Officials in several countries have denied or rejected allegations from the leaked records.

Kremlin spokesperson Dmitry Peskov told Sky News: "Like everyone else, we don't know the degree of authenticity of these documents.

"But even if we disengage from them, long before these documents appeared, we had and still have information that many instructors, including the ones from Britain, and fighters are taking part in the military actions."