The FBI was concerned President Donald Trump was working in coordination with Russia during his first term when he fired former FBI Director James Comey, newly revealed documents show.

According to White House official John Solomon, the probe, dubbed Oxferd Comma, investigated why Trump fired Comey in 2017.

The effort was a subsection of the wider Crossfire Hurricane probe the FBI had begun to investigate potential collusion between Trump's 2016 campaign and the Russian government. 

Trump has long railed against the investigation into his campaign and Russia, repeatedly calling it a hoax and a witch hunt. 

After Trump fired Comey in May 2017, the FBI grew suspicious that the President could be obstructing justice, which then led to former Special Counsel Robert Mueller's appointment and investigation into the President. 

Trump said that he fired Comey over his handling of former Secretary of State Hillary Clinton's email scandal that year. 

Still, some FBI officials believed that the Republican fired the director potentially at the behest of Russia. 

'But previously released FBI and DOJ records show the notion of Trump being a Russian asset had already been widely dismissed by agents and analysts working for months beforehand on the Crossfire Hurricane case that examined Russia-Trump collusion,' Solomon wrote in a White House report. 

Oxferd Comma was reportedly marked sensitive, a designation that could help keep the probe off the books and therefore make it more difficult for other agents to find. 

It was eventually rolled into Mueller's investigation.

However, once the Mueller investigation was concluded and Trump was not implicated, an official told CBS News that Oxferd Comma's probe lacked merit.  

'I was unaware that the predicate for the obstruction included a claim that Donald Trump was somehow acting on behalf of Vladimir Putin,' the official told CBS News.

'The short of it is, almost everything in the FBI's possession at that point had debunked or ruled out that Trump or his campaign was colluding with Russia at that time.'

Earlier this year, Comey was charged by the Department of Justice (DOJ) for making threats against the President. 

The former FBI Director had posted a picture on his social media with the slogan '86 47' spelled out using seashells. 

The phrase '86' is known in some circles to mean 'get rid of,' which the President has taken as a threat. 

It is the second time the DOJ has tried a case against Comey for the post after a Virginia-based case was thrown out last year.