Waltz was photographed using Signal during Cabinet meeting a day before his removal

Waltz was at the center of the Signal chat about a strike on Houthis in Yemen.

President Donald Trump's former national security adviser Mike Waltz was photographed using Signal during a Cabinet meeting on Wednesday, just one day before the president announced he was replacing Waltz with Secretary of State Marco Rubio.

In the photo, taken by a Reuters photographer, the names of people Waltz had been texting included Vice President JD Vance, Rubio, Director of National Intelligence Tulsi Gabbard, and Steve Witkoff, Trump's Middle East envoy.

White House Communications Director Steven Cheung defended Waltz's use of the app on Thursday afternoon, saying, "Signal is an approved app that is loaded onto our government phones. Thank you for your attention to this matter."

Waltz came under fire in March after he inadvertently invited a journalist, The Atlantic's Jeffrey Goldberg, to a Signal group chat with other national security officials, such as Vance and Defense Secretary Pete Hegseth, where they discussed a U.S. attack on terrorists in Yemen.

Waltz claimed he did not know how Goldberg got on the chat and claimed no sensitive information was shared. Trump defended Waltz and brushed off calls to fire him.

On Thursday, Trump announced he would nominate Waltz to be U.S. ambassador to the United Nations, while Secretary of State Marco Rubio will serve as interim national security adviser while keeping his current role as well.

Though Trump publicly backed Waltz, sources told ABC News that the president has been increasingly frustrated with him after he came under intense scrutiny over the Signal controversy.

Despite the administration’s position that Signal is an approved app, the Pentagon's internal watchdog criticized a former official's use of the Signal app in 2021, calling it a breach of the department's "records retention policies" and an unauthorized means of communicating sensitive information.

Last month, DOD acting Inspector General Steven Stebbins announced he was starting an investigation into Hegseth's use of Signal during the Yemen attack. On Thursday a U.S. official confirmed to ABC News that the IG was looking into a second Signal chat in which Hegseth shared timing for the attack with his wife, brother and attorney.

The Wall Street Journal forst reported on the expanded investigation.

"The objective of this evaluation is to determine the extent to which the Secretary of Defense and other DoD personnel complied with DoD policies and procedures for the use of a commercial messaging application for official business. Additionally, we will review compliance with classification and records retention requirements," he said in a statement.

Republican leaders have blocked Democrats' efforts to investigate the Signal chat concerning the Yemen attack in Congress.