President Donald Trump's son-in-law and White House advisor Jared Kushner used the encrypted messaging service WhatsApp to conduct official business, according to a report in the New York Times.
"For example during a meeting with Mr. Kushner's attorney, Abbe Lowell, Mr. Lowell confirmed that Mr. Kushner has been using the messaging application WhatsApp as part of his official White House duties to communicate with foreign leaders".
Senior adviser to the President Jared Kushner used private email and a messaging app to conduct official business, the House oversight committee says. When asked, Lowell said, "That's above my pay grade", and referred questions to the White House and the National Security Council, according to Cummings' letter.
In the case that an employee must conduct official business on a personal account, he or she must forward a copy of the communication to the official account and delete it from the personal account.
The request will go to the Department of Justice, the Federal Bureau of Investigation and the White House counsel's office, among others, the Post said, citing unnamed congressional aides familiar with the plan. "I've never heard that, I've never heard about it", the Republican president said.
A representative for Bannon did not immediately respond to a request for comment. Cummings says that appears to violate the Presidential Records Act.
Aside from the Kushners, the House committee said other officials such as the former deputy national security adviser K.T. McFarland and former White House chief strategist Steve Bannon used their personal email accounts to talk about the "transfer of sensitive US nuclear technology to Saudi Arabia".
The revelations are particularly embarrassing for the president, who during the 2016 campaign called for the jailing of Hillary Clinton over her own use of a private email account and server to exchange top secret information.
"If you continue to withhold these documents from the Committee", Cummings added, "we will be forced to consider alternative means to obtain compliance", a not-so-veiled allusion to a possible subpoena.
Last year, Ivanka Trump alleged "there's no connection between" her email use and Clinton's.
"Presidents from all political parties have determined that the law does not require the Executive Branch to provide Congress with documents relating to confidential diplomatic communications between the President and foreign leaders ..."
In a joint statement on Thursday night, Mr Cummings, Mr Engel and Mr Schiff said that the Obama administration had "produced records describing the president and secretary of state's calls with foreign leaders".