WASHINGTON (AP) -- President Donald Trump on Tuesday downplayed the texting of sensitive plans for a military strike against Yemen's Houthis this month to a group chat that included a journalist, saying it was "the only glitch in two months" of his administration as Democratic lawmakers heaped criticism on the administration for handling highly sensitive information carelessly.

Trump told NBC News that the lapse "turned out not to be a serious one," and articulated his continued support for national security adviser Mike Waltz, who mistakenly added the editor-in-chief of The Atlantic, Jeffrey Goldberg, to the chain that included 18 senior administration officials discussing planning for the strike.

"Michael Waltz has learned a lesson, and he's a good man," Trump said in the NBC interview. The president also appeared to point blame on an unnamed Waltz aide for Goldberg being added to the chain. "It was one of Michael's people on the phone. A staffer had his number on there."

But the use of messaging app Signal to discuss a sensitive operation has opened the administration to blistering criticism from Democratic lawmakers who expressed outrage at the White House's and senior administration officials' insistence that no classified information was shared. Senior administration officials have struggled to explain why the publicly available app was used to discuss such a delicate matter.

One official on the Signal chain, Director of National Intelligence Tulsi Gabbard, acknowledged during a Senate Intelligence Committee hearing on Tuesday that she was traveling overseas during the exchange. She wouldn't say whether she was using her personal or government-issued phone because the matter is under review by the White House National Security Council.

Both Gabbard and CIA Director John Ratcliffe, who also was a participant in the Signal exchange and also testified at Tuesday's intelligence hearing, faced blistering criticism from lawmakers.

"This is an embarrassment," said Sen. Jon Ossoff, a Georgia Democrat. "This is utterly unprofessional. There's been no apology. There's been no recognition of the gravity of this error."

In the run-up to his 2016 election victory over Democrat Hillary Clinton, Trump urged criminal prosecution of the former secretary of state for communicating about classified information with her aides on a private email server she set up. The matter was investigated, but the FBI ultimately recommended against charges. None were brought.

Clinton was among Democrats this week to criticize Trump administration officials' use of Signal.

"You have got to be kidding me," Clinton said in an X post that spotlighted The Atlantic article and included an eyes emoji.

Trump also faced charges for mishandling classified information at his Mar-a-Lago resort following his first White House term. Those charges were later dismissed.

But on Tuesday, top administration officials were insistent the Democratic outrage about the matter was misplaced.

On Capitol Hill, Ratcliffe and Gabbard told lawmakers that no classified information was included in the texts about U.S. attack plans in the message chain.

Democrats pushed back, saying the leaked military plans show a sloppy disregard for security, but Ratcliffe insisted no rules were violated.

"My communications to be clear in the Signal message group were entirely permissible and lawful and did not include classified information," Ratcliffe told lawmakers in the hearing that was supposed to be focused on global security threats.

Facing heated questions from Democratic Sen. Mark Warner of Virginia, Gabbard said there's a difference between "inadvertent" releases of information and intentional leaks. "There was no classified material that was shared," Gabbard said.

Warner, though, said the lapse in security could have cost lives.

"If this information had gotten out, American lives could have been lost. If the Houthis had this information they could reposition their defensive systems," Warner said.

In response to questions from Democratic Sen. Ron Wyden of Oregon, Gabbard and Ratcliffe said they would participate in an audit looking into administration officials' use of Signal. Wyden said the matter must be investigated.

"I'm of the view that there ought to be resignations," Wyden said.

FBI Director Kash Patel, appearing with Ratcliffe and Gabbard at the hearing, said he was only recently briefed on the Signal chat matter and doesn't have an update on whether the FBI has opened an investigation into it. Warner asked for an update by the end of the day.

The White House in a statement Tuesday called the uproar a "coordinated effort to distract from the successful actions taken by President Trump and his administration to make America's enemies pay and keep Americans safe."

Defense Secretary Pete Hegseth, in his first comments on the matter on Monday, attacked the journalist who received the messages, Goldberg, as "deceitful" and a "discredited so-called journalist" while alluding to previous critical reporting of Trump from the publication.

"Nobody was texting war plans and that's all I have to say about that," Hegseth said in an exchange with reporters on Monday.

Signal is an app that can be used for direct messaging and group chats as well as phone and video calls. It uses end-to-end encryption for its messaging and calling services that prevents any third party from viewing conversation content or listening in on calls.

In other words, messages and calls sent on Signal are scrambled; only the sender and recipient at each end will have the key to decipher them.

Signal's encryption protocol is open source, meaning that it's freely available for anyone to inspect, use or modify. The encryption protocol is also used by another popular chat service, social media company Meta's WhatsApp platform.

Government officials have used Signal for organizational correspondence, such as scheduling sensitive meetings. But in the Biden administration, people who had permission to download it on their White House-issued phones were instructed to use the app sparingly, according to a former national security official who served in the administration.

The official, who requested anonymity to speak about methods used to share sensitive information, said Signal was most commonly used to notify someone that they should check for a classified message sent through other means.

The use of Signal became more prevalent during the last year of the Biden administration after federal law enforcement officials warned that China and Iran were hacking the White House as well as officials in the first Trump administration, according to the official.

Sen. Angus King, a Maine Independent, questioned Ratcliffe and Gabbard over their assertion that no classified information was included in the chat.

"It's hard for me to believe that targets and timing and weapons would not have been classified," he said.

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AP writer Kelvin Chan in London contributed reporting.