The US intelligence community's watchdog rejected a theory floated by President Donald Trump that the whistleblower process was changed just before an explosive complaint about him.

The complaint alleged that Trump pressured Ukraine's president to investigate Joe Biden in a phone call. It is largely based on the whistleblower's sources.

Trump and his allies argue that the process was only recently changed to allow complaints from people without first-hand information .

The Inspector General of the Intelligence Committee said Monday that first-hand information was never been a requirement, and that the current process has existed for more than a year.

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The intelligence community's watchdog poured cold water on a claim by President Donald Trump that the rules for whistleblower complaints were changed just before a an explosive accusation was lodged about his dealings with Ukraine.

The complaint drew attention to a phone call between Trump and Ukrainian President Volodymyr Zelensky, which the official memo released by the White House showed Trump used to ask Zelensky to investigate Joe Biden and his son.

Details about the call led Democrats to launch an impeachment inquiry into Trump.

Ukraine's President Volodymyr Zelensky meets US President Donald Trump at the United Nations General Assembly in New York City on September 25, 2019. REUTERS/Jonathan Ernst

On Monday, Trump angrily tweeted the suggestion that the rules had been changed just before the complaint hit, seeming to imply that standards had been lowered in order to admit the case against him.

Trump wrote: "WHO CHANGED THE LONG STANDING WHISTLEBLOWER RULES JUST BEFORE SUBMITTAL OF THE FAKE WHISTLEBLOWER REPORT? DRAIN THE SWAMP!"

He was echoing claims made by GOP senator Lindsey Graham, who said on Sunday: "I want to know why they changed the rules about whistleblowers not — the hearsay rule was changed just a short period of time before the complaint was filed."

Republican Senators Chuck Grassley, Mike Lee, and Ron Johnson also raised the possibility of the rules having changed on Monday, though with less certainty than Trump and Graham.

In response, officials for Michael Atkinson, the Inspector General of the Intelligence Committee, released a four-page statement on Monday shooting down the theory. It did not name Trump or any of the senators.

The statement said that having first-hand knowledge has never been a requirement, that the whistleblower used a process that has been in place for more than a year, and added that the whistleblower also does claim to have first-hand information.

Read more: The whistleblower at the center of the Trump impeachment inquiry has agreed to testify before Congress, says Rep. Adam Schiff

It said: "Although the form requests information about whether the Complainant possesses first-hand knowledge about the matter about which he or she is lodging the complaint, there is no such requirement set forth in the statute."

President Donald Trump speaks to member of the media in the Oval Office of the White House on Monday. Associated Press

"In fact, by law the Complainant – or any individual in the Intelligence Community who wants to report information with respect to an urgent concern to the congressional intelligence committees – need not possess first-hand information in order to file a complaint or information with respect to an urgent concern."

The statement said that the process the whistleblower used on August 12, 2019, has been in place since May 2018.

Read more: 2 key facts refute Trump's conspiracy theories about the Ukraine scandal

It suggested that a misunderstanding about the process could have come from other, different, whistleblower processes which were not used in this case. They also do not require first-hand information, but the language in the forms could be misunderstood to suggest that they do, it said.

Trump has called the complaint "fake" and "a second-hand description" in defending himself from the allegations it contains. He maintains that he did nothing wrong in the call with Zelensky.

Barr was 'surprised and angry' to discover Trump had lumped him in with Giuliani during his call with the Ukrainian president

Senate Republicans have barely challenged Trump over Ukraine. Don't expect them to turn against him in an impeachment trial.

The White House reportedly tried to conceal transcripts of Trump's calls with other world leaders, including Russia's Putin and Saudi Arabia's Mohammad bin Salman

Trump suggested the whistleblower who filed a complaint against him is guilty of treason, which is punishable by death