The chairman, Representative Elijah E. Cummings of Maryland, said that a lawyer for Ms. Trump, President Trump’s daughter, and Mr. Kushner, the president’s son-in-law and senior adviser, told the committee late last year that in addition to a private email account, Mr. Kushner uses an unofficial encrypted messaging service, WhatsApp, for official White House business, including with foreign contacts.Mr. Cummings said the lawyer, Abbe Lowell, also told lawmakers that Ms. Trump did not preserve some emails sent to her private account if she did not reply to them.