Members of President Donald Trump’s National Security Council, including White House national security adviser Michael Waltz, have conducted government business over personal Gmail accounts, according to documents reviewed by The Washington Post and interviews with three U.S. officials.

The use of Gmail, a far less secure method of communication than the encrypted messaging app Signal, is the latest example of questionable data security practices by top national security officials already under fire for the mistaken inclusion of a journalist in a group chat about high-level planning for military operations in Yemen.

A senior Waltz aide used the commercial email service for highly technical conversations with colleagues at other government agencies involving sensitive military positions and powerful weapons systems relating to an ongoing conflict, according to emails reviewed by The Post. While the NSC official used his Gmail account, his interagency colleagues used government-issued accounts, headers from the email correspondence show.......

Data security experts have expressed alarm that U.S. national security professionals are not more readily using the government’s suite of secure encrypted systems for work communications such as JWICS, the Joint Worldwide Intelligence Communications System.

Most concerning, however, is the use of personal email, which is widely acknowledged to be susceptible to hacking, spearfishing and other types of digital compromise.

“Unless you are using GPG, email is not end-to-end encrypted, and the contents of a message can be intercepted and read at many points, including on Google’s email servers,” said Eva Galperin, director of cybersecurity at the Electronic Frontier Foundation.

National security experts have expressed alarm over the administration’s denial that the leaked Signal chat contained classified information.