In September 2016, then-candidate Donald Trump, his son Donald Trump Jr, and others were offered a decryption key to gain access to leaked Wikileaks documents ahead of the general public.
The email was provided to investigators working on the Congressional inquiry into alleged collusion between the Trump campaign team and Russian officials during the 2016 US election.
The email was sent on 4 September and Congress is attempting to figure out whether the email is proof of a legitimate attempt by Wikileaks to contact the Trump campaign, CNN reported.
There is no indication whether any of the recipients acted on the email.
Mr Trump Jr has repeatedly denied wrongdoing and during testimony in front of the House Intelligence Committee on 8 December said he did not remember receiving this particular email.
Just one month before, the hacked emails from the Democratic National Committee (DNC) server were released. One month later Wikileaks would begin leaking emails from John Podesta, Hillary Clinton’s campaign chairman.
Congress is also unable to identify the sender of the email, someone calling themselves "Mike Erickson".
This person addressed the email to Mr Trump Jr, his personal assistant, and Mr Trump - who has famously said he does not use email.
It is not an uncommon for Wikileaks to post a file of leaked documents online, but lock it so that only people with the specific decryption key can access it.
Adding to the controversy over this particular email is that Mr Trump Jr has had prior contact with Wikileaks, by his own admission.
He released direct messages on Twitter he had exchanged with Wikileaks after a report from The Atlantic magazine brought the communications to light.
Appropriately, he tweeted: "Here is the entire chain of messages with @wikileaks (with my whopping 3 responses) which one of the congressional committees has chosen to selectively leak. How ironic!"
Here is the entire chain of messages with @wikileaks (with my whopping 3 responses) which one of the congressional committees has chosen to selectively leak. How ironic! 1/3 pic.twitter.com/SiwTqWtykA
— Donald Trump Jr. (@DonaldJTrumpJr) November 14, 2017
Congress is trying to figure out whether there is a link between those communications and the decryption key email.
Mr Trump Jr, according to sources who spoke to CNN, said the messages he shared were akin to speaking to any major media outlet.
The decryption email is just the latest in a string of revelations in the investigation into alleged collusion - conducted in parallel by Congress and the FBI with special prosecutor Robert Mueller.
Former campaign manager Paul Manafort, deputy Rick Gates, and aide George Papadopoulos have all been indicted already.
Mr Manafort and Mr Gates were brought up on unrelated fraud and financial crimes and Mr Papadopoulos pleaded guilty to one count of lying to the FBI about his contacts with Russia-linked individuals.
On 1 December, former National Security Adviser Michael Flynn pleaded guilty to lying to the FBI as well. He was indicted for making false statements about the nature of a 29 December 2016 conversation he had with former Russian Ambassador to the US Sergey Kislyak.
Both investigations are continuing.
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- Donald Trump
- Donald Trump Jr
- WikiLeaks
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- 2016 US election
