Sally Yates has taken to Twitter to say Donald Trump's "refusal" to believe Russians hacked the US election mean it could happen again.
The former Deputy Attorney General, who was sacked by Mr Trump when she was Acting Attorney General, appears to be referring to the President's comments during his visit to Poland.
In a news conference with Polish President Andrzej Duda, Mr Trump said: "I think it was Russia and I think it could have been other people and other countries. It could have been [that] a lot of people interfered."
He did not name any other countries that could have possibly hacked the US election, however, adding: "Nobody really knows for sure."
He also called into question a January 2017 report issued by the US intelligence community that confirmed Russian interference in the 2016 election. Original news reports said all 17 agencies had signed on to the report.
Mr Trump, in Poland, said that only "three of four" agencies had done so and called reports about it "fake news".
It is true that the main agencies that said they have "high confidence" of Russian hacking were the Central Intelligence Agency, the National Security Agency, the F.B.I. and the Office of the Director of National Intelligence (DNI). However, the New York Times characterised the President's comments as "misleading".
The biggest names involved in the Trump-Russia investigation
Paul Manafort
Mr Manafort is a Republican strategist and former Trump campaign manager. He resigned from that post over questions about his extensive lobbying overseas, including in Ukraine where he represented pro-Russian interests. GettyMike Flynn
Mr Flynn was named as Trump's national security adviser but was forced to resign from his post for inappropriate communication with Russian Ambassador to the US Sergey Kislyak. He had misrepresented a conversation he had with Mr Kislyak to Vice President Mike Pence, telling him wrongly that he had not discussed sanctions with the Russian. Getty ImagesSergey Kislyak
Mr Kislyak, the Russian ambassador to the US, is at the centre of the web said to connect President Donald Trump's campaign with Russia. ReutersRoger Stone
Mr Stone is a former Trump adviser who worked on the political campaigns of Richard Nixon, George HW Bush, and Ronald Reagan. Mr Stone claimed repeatedly in the final months of the campaign that he had backchannel communications with WikiLeaks founder Julian Assange and that he knew the group was going to dump damaging documents to the campaign of Democratic candidate Hillary Clinton - which did happen. Mr Stone also had contacts with the hacker Guccier 2.0 on Twitter, who claimed to have hacked the DNC and is linked to Russian intelligence services. Getty ImagesJeff Sessions
The US attorney general was forced to recuse himself from the Trump-Russia investigation after it was learned that he had lied about meeting with Russian Ambassador to the US Sergey Kislyak. Getty ImagesCarter Page
Mr Page is a former advisor to the Trump campaign and has a background working as an investment banker at Merrill Lynch. Mr Page met with Russian Ambassador to the US Sergey Kislyak during the 2016 Republican National Convention in Cleveland. Mr Page had invested in oil companies connected to Russia and had admitted that US Russia sanctions had hurt his bottom line. ReutersJeffrey "JD" Gorden
Mr Gordon met with Russian Ambassador to the US Sergey Kislyak during the 2016 Republian National Convention to discuss how the US and Russia could work together to combat Islamist extremism should then-Republican presidential candidate Donald Trump win the election. The meeting came days before a massive leak of DNC emails that has been connected to Russia. Creative CommonsJared Kushner
Mr Kushner is President Donald Trump's son-in-law and a key adviser to the White House. He met with a Russian banker appointed by Russian President Vladimir Putin in December. Mr Kushner has said he did so in his role as an adviser to Mr Trump while the bank says he did so as a private developer. Mr Kushner has also volunteered to testify in the Senate about his role helping to arrange meetings between Trump advisers and Russian Ambassador to the US Sergey Kislyak. Getty ImagesJames Comey
Mr Comey was fired from his post as head of the FBI by President Donald Trump. The timing of Mr Comey's firing raised questions around whether or not the FBI's investigation into the Trump campaign may have played a role in the decision. Getty ImagesPreet Bharara
Mr Bahara refused, alongside 46 other US district attorney's across the country, to resign once President Donald Trump took office after previous assurances from Mr Trump that he would keep his job. Mr Bahara had been heading up several investigations including one into one of President Donald Trump's favorite cable television channels Fox News. Several investigations would lead back to that district, too, including those into Mr Trump's campaign ties to Russia, and Mr Trump's assertion that Trump Tower was wiretapped on orders from his predecessor. Getty ImagesSally Yates
Ms Yates, a former Deputy Attorney General, was running the Justice Department while President Donald Trump's pick for attorney general awaited confirmation. Ms Yates was later fired by Mr Trump from her temporary post over her refusal to implement Mr Trump's first travel ban. She had also warned the White House about potential ties former National Security Adviser Michael Flynn to Russia after discovering those ties during the FBI's investigation into the Trump campaign's connections to Russia. Getty ImagesThe DNI serves as an umbrella coordinating agency for the massive, widespread US intelligence community. Also, "there is no evidence that significant uncertainty or dissent exists across the intelligence community, simply because not all 17 were involved in the assessment of Russian interference" as the newspaper reported.
However there is a simpler explanation - those four agencies were doing the work to track Russian hacking into the election while the other 13 agencies may have been doing other work or only supplementary work on the issue.
Ms Yates, who has tweeted once thus far, said the President's attitude "insults" career intelligence professionals:
Ms Yates believes she was fired early in Mr Trump's term, not because she was an appointee of Barack Obama but because she warned White House officials about then-National Security Adviser Mike Flynn's alleged ties to Russian officials.
She told them that Mr Flynn was at risk for blackmail and that he had misled senior officials including Vice President Mike Pence about his dealings with Russian Ambassador to the US Sergei Kislyak before the Trump administration came into office.
Mr Trump is currently meeting with Russian President Vladimir Putin at the G20 summit in Hamburg, Germany.
There has been no set agenda communicated to the public; the White House has said the conversation will be about "whatever the president wants to talk about".
There is no indication Mr Trump will bring up the hacking or the ongoing FBI, House, Senate, and Special Prosecutor Robert Mueller's investigations into alleged ties between the Trump campaign team and Russian officials.