Senior White House officials tried to “lock down” details of a phone call between Donald Trump and the Ukrainian president, a whistleblower, whose identity has not been disclosed, alleged in a fresh complaint on Thursday, reported BBC. On July 25, Trump, during a phone call, had pushed President Volodymyr Zelenskiy to investigate Joe Biden and his son Hunter. Biden is a contender for the Democratic candidate in the upcoming US presidential election.

The whistleblower said the call transcript was not stored in the usual computer. The transcript was stored in a separate system “that is otherwise used to store and handle classified information of an especially sensitive nature”, wrote the whistleblower. “This set of actions underscored to me that White House officials understood the gravity of what had transpired in the call.” The whistleblower also alleged that Trump’s actions on Ukraine “pose risks to US national security”, reported The Guardian.

According to The New York Times, the whistleblower is an officer of the Central Intelligence Agency.

On Wednesday, Trump released a memo of the July 25 call to Zelenskiy. “There’s a lot of talk about Biden’s son, that Biden stopped the prosecution and a lot of people want to find out about that, so whatever you can do with the attorney general would be great,” Trump said, according to the memo of the call.

Trump released aid to Ukraine in September, after withholding it for about two months. This sparked speculation that he had withheld the money to put more pressure on the Ukraine president. In a press conference following the release of the transcript, Trump denied that he tried to pressure Ukraine’s president “to do things that they wanted under the form of political threat”. He said he did not threaten anyone.

On Tuesday, House of Representatives Speaker Nancy Pelosi had announced a formal impeachment inquiry against Trump. “Today, I am announcing the House of Representatives moving forward with an official impeachment inquiry,” Pelosi said at Capitol Hill. “The president must be held accountable. No one is above the law. Actions taken to date by the president have seriously violated the Constitution.”

Pelosi’s announcement of an impeachment investigation has been backed by several House Democrats. Senator Hillary Clinton, who lost the 2016 presidential election to Trump, also favoured impeachment. “The president of the United States has betrayed our country,” she tweeted. “That’s not a political statement – it’s a harsh reality, and we must act. He is a clear and present danger to the things that keep us strong and free. I support impeachment.

Trump claimed that the Democratic Party was on a witch hunt, and that his call to the Ukrainian president was a “very friendly and totally appropriate call”.

After the whistleblower’s complaint, members of Ukraine’s civil society have raised questions about how trustworthy an ally the United States could be, reported The Guardian. “This complaint confirms all the worries that we had,” said Daria Kaleniuk, the executive director of Ukraine’s Anti-Corruption Action Centre. “This is very strange to have this pressure from a longtime ally of Ukraine, a request to interfere in an investigation from a country which was a role model for us for a long time.