On Christmas Day, Michael Flynn sent a simple text.

"Merry Christmas and a Happy New Year," the man designated to become US national security adviser wrote to Russian ambassador Sergey Kislyak, and got a reply wishing him the same in return.

Four days later, the US slapped sanctions on Russian intelligence services and kicked 35 officials out of the country in retaliation for alleged interference in the election campaign.

Again, Flynn texted the ambassador and the two spoke on the phone. The exact details of that call are now at the centre of a national security scandal that cost Flynn his job.

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Overnight, US President Donald Trump blasted "fake news media" on Twitter for "going crazy with their conspiracy theories and blind hatred". He fumed at the leaks coming out of Washington that have damaged his White House and led to the sacking of a trusted adviser just one month into the job.

Despite Trump's outrage, the story is not going away. University College London Professor Iwan Morgan said it raised questions that cut to the heart of an "old-fashioned turf war" between intelligence services, the White House and the role of Russia in the world. Among them is the "really dangerous" one for Trump: What did he know about Flynn's conversation and when?

"If it comes out he sent some message ... [to Russia, and] did that in secret rather than out in the open, that's problematic," he said.

"The real threat is that Congress will pick up on this ... As the heat builds there will be a drip, drip, drip [of intelligence leaks]. This story is going to run and run unless he goes on national television to lance the boil. He's got to address the American people, not go on Fox News or tweet it."

Here's what you need to know about the scandal that is not going away.

WHAT HAPPENED WITH FLYNN?

The texts and calls between Flynn and Kislyak are in the spotlight because they could violate the Logan Act, which prohibits private citizens from interfering with foreign governments unless given special permission. They also cut to the heart of a much wider question about whether or not Russia has any influence over Trump's team, which has been bubbling under the surface for months.

After the Christmas text message exchange, Putin uncharacteristically said he would not retaliate against the sanctions ordered by then president Barack Obama over hacking. Instead he would reset the relationship based on "policies that will be carried out by the administration of President Trump".

Trump later praised Putin on Twitter saying the delay was "great move". "I always knew he was very smart" he said.

When news of the phone conversation came to light, Flynn said it was designed to discuss the logistics of setting up a call between Trump and Putin - a line that was repeated by Vice-President Mike Pence and White House spokesman Sean Spicer on multiple occasions.

Former attorney-general Sally Yates warned the White House there was more to Flynn's conversations than he let on initially. Photo / AP
Former attorney-general Sally Yates warned the White House there was more to Flynn's conversations than he let on initially. Photo / AP

But in late January, acting attorney-general Sally Yates (

) told the White House that intelligence had intercepted the call and knew Flynn was lying about its contents. She also feared he could be vulnerable to blackmail. The White House said Trump and senior advisers were "immediately" informed and legal counsel asked to investigate.

Despite the warning, it wasn't until last Thursday that Flynn changed his tune and said he couldn't remember the content of the conversation. On Monday, The Washington Post reported that the Justice Department had warned Trump weeks earlier and Flynn resigned from his post.

SEPARATE CLAIMS OF CONTACT WITH RUSSIA

The Flynn drama was swiftly followed by another report from The New York Times claiming members of Trump's team had been in contact with senior Russian intelligence officers in the year leading up to the election.

Citing four current and former intelligence officials, the paper claimed the contact was discovered during an investigation of Democratic party hacks during the campaign.

It said a classified investigation was under way but there was no evidence the Trump team was collaborating with Russia. Trump's team has consistently denied any collaboration.

It's also possible members of the team came into contact with Russian intelligence operatives without realising it.

In this image made from a video taken on December 10, 2015 Donald Trump's former National Security Adviser Michael Flynn, right, shakes hands with Vladimir Putin.
In this image made from a video taken on December 10, 2015 Donald Trump's former National Security Adviser Michael Flynn, right, shakes hands with Vladimir Putin.

WHY DOES IT MATTER?

Though separate, Flynn's resignation and the New York Times report point to a much wider theme that has been troubling the intelligence community and Democrats for months: How much influence, if any, does Russia have on members of the Trump administration?

Flynn's case is significant because his role as national security adviser means he is privy to some of the world's most critical military, intelligence, terror-related and diplomatic information. It also comes at a time of increased military tension globally, with the North Atlantic Treaty Organisation (NATO) building up forces on the Russian border and vice-versa.

Trump has consistently denied having any contact with Russia, blaming the Democrats for being sore losers.

The Kremlin has also shut down any possibility of a link, saying unnamed sources can't be trusted.

"Don't believe newspaper reports. It's very difficult at the moment to differentiate them from falsehoods and fabrications," Kremlin spokesman Dmitry Peskov said.

"If you don't mind, let's wait and let's not believe anonymous information, which is information based on no fact."

WHAT HAPPENS NEXT?

Prof Morgan said it was "too early" to tell how the scandal would unfold but we were likely to see a continued string of intelligence leaks given that agencies Trump blasted were likely to feel that they didn't have the ear of the President.

The US expert at London policy institute Chatham House, Dr Jacob Parakilas, said whether or not Congress opted to expand its investigation would be crucial.

"This really isn't the end of the story," he said. "Thus far, the [White] House has been very resistant to meaningful investigation into the matter, the Senate somewhat less so.

"But if the pace at which new revelations are emerging, the pressure on Congress to broaden the scope of its current investigations will only increase, and the extent to which it does so will dictate where the story goes from here."