According to a criminal indictment that was only unsealed after the WikiLeaks founder's arrest, Assange is charged with conspiring with former army intelligence analyst Chelsea Manning to gain unauthorised access to a government computer.
The US indictment filed in March 2018 said Assange in March 2010 engaged in a conspiracy to help Manning crack a password stored on Defense Department computers connected to the Secret Internet Protocol Network (SIPRNet), a US government network used for classified documents and communications.
Assange's contacts with Manning led to one of the biggest ever leaks of classified information as WikiLeaks published hundreds of thousands of US military reports and diplomatic communications.
The US official said that within the 60-day period, US authorities could modify or add to the current charges they have filed against Assange.
The official declined to say whether further charges were likely, but legal experts have said they are certainly possible.
A witness who prosecutors were seeking to interview and an associate of Assange based in Europe - who also requested anonymity - said that before his arrest Assange worried that US prosecutors would also bring charges related to WikiLeaks' publication of CIA computer hacking tools, which the website described as its "Vault 7" cache.
US officials have said that the disclosure of details about the US spy agency's abilities to perform electronic surveillance and cyber warfare was potentially far more damaging to government activities than anything Manning made available to WikiLeaks.
In a Friday interview with CNN, US Vice President Mike Pence said the United States was "going to bring Julian Assange to justice".
Pence denied that statements by President Donald Trump, in which he praised WikiLeaks during the 2016 US election campaign, were in any way an endorsement of the organisation.
Meanwhile, the lawyer of the Swedish woman who accused Julian Assange of rape in 2010 said Thursday she and her client would ask Swedish prosecutors to reopen the investigation which was dropped in 2017.
"We will do everything we can to get the prosecutors to reopen the Swedish investigation so that Assange can be extradited to Sweden and be prosecuted for rape. As long as the statute of limitations has not expired my client has hope for restitution," lawyer Elisabeth Massi Fritz told AFP.
Sweden's director of public prosecutions Marianne Ny decided in May 2017 to shut a preliminary investigation into the rape allegations.
She argued that since Assange could not be reached after taking up residence in the Ecuadorian embassy in London in 2012, it was not possible to proceed with the probe.
Ny also said that if Assange were to become available again, prosecutors could decide to reopen the investigation.
A spokesman for the Swedish Prosecution Authority said they were following the news of Assange's arrest.
"We are looking into the issue," he said.
The Swedish accusation against Assange dates from August 2010 when the alleged victim, who says she met him at a WikiLeaks conference in Stockholm a few days earlier, filed a complaint.
She accused him of having sex with her - as she slept - without using a condom despite repeatedly having denied him unprotected sex.
Assange has always denied the allegations, which he feared would lead to him being extradited to the United States and facing trial over the leak of hundreds of thousands of secret US military and diplomatic documents in 2010, which brought WikiLeaks to prominence.