The complaint concerned a "promise" Trump made in a recent phone call with a foreign leader.

A mysterious whistleblower, a telephone call between President Donald Trump and a foreign leader, and military aid to Ukraine.

Here's what we know about the latest political firestorm swirling around Washington:

Whistleblower

On August 12, an unknown whistleblower in the US intelligence community filed a complaint with Michael Atkinson, the Inspector General of the Intelligence Community (ICIG), regarding a matter of "urgent concern."

The identity of the whistleblower and the nature of the complaint have not been publicly revealed.

After reviewing the complaint, Atkinson determined it was "credible" and forwarded it on August 26 to the acting Director of National Intelligence, Joseph Maguire, a Trump appointee.

The Democratic-led House Intelligence Committee is demanding that the whistleblower's complaint be turned over but Maguire has declined to share it.

According to The Washington Post, the complaint concerned a "promise" Trump made in a recent phone call with a foreign leader.

Phone call

The phone call in question, according to US media reports, was a July 25 conversation between Trump and Ukraine's newly elected President Volodymyr Zelensky.

According to The Wall Street Journal, Trump repeatedly pressured Zelensky in the call to investigate Hunter Biden, the son of Joe Biden, Trump's main potential 2020 reelection rival.

Hunter Biden in 2014 joined the board of Burisma Holdings, a Ukrainian gas firm accused of corrupt practices, but he has not been personally accused of any wrongdoing.

The Journal said Trump urged Zelensky "about eight times" during the call to work with Rudy Giuliani, his personal attorney, on a probe into Hunter Biden.

At the time of the conversation, the United States was holding up $250 million in military aid for Ukraine, but the Journal said Trump did not mention the assistance during the conversation.

Three committees in the Democratic-controlled House of Representatives have demanded that the White House turn over a transcript of the call.

The committees claim it may include evidence of Trump and Giuliani's efforts to "improperly pressure" Ukraine to help his reelection bid.

They accused Trump and Giuliani of trying to coerce Ukraine, in exchange for military assistance, into conducting a "politically motivated" investigation into Hunter Biden.

"If the President is trying to pressure Ukraine into choosing between defending itself from Russian aggression without US assistance or leveraging its judicial system to serve the ends of the Trump campaign, this would represent a staggering abuse of power," the Democratic committee chairmen said.

The US military aid package for Ukraine, which is battling pro-Russian separatists, was finally approved by the White House last week.

Trump and Giuliani fight back

Trump for his part has insisted there was nothing untoward about the telephone call.

"Virtually anytime I speak on the phone to a foreign leader, I understand that there may be many people listening from various U.S. agencies, not to mention those from the other country itself," he tweeted on Thursday.

"Knowing all of this, is anybody dumb enough to believe that I would say something inappropriate with a foreign leader while on such a potentially 'heavily populated' call," he said. "I would only do what is right anyway, and only do good for the USA!"

Speaking to reporters at the White House on Friday, Trump described the story as a "political hack job" and said former vice president Joe Biden is the one who should be investigated.

"It was a totally appropriate conversation," he said of the telephone call. "It was actually a beautiful conversation."

Trump claimed he didn't know the identity of the whistleblower but said he was a "partisan person."

Giuliani, the former mayor of New York, also took to Twitter to defend Trump.

"A President telling a Pres-elect of a well known corrupt country he better investigate corruption that affects US is doing his job," he tweeted.