(CNN) — US officials reacted with shock—and in many cases, horror—to revelations in The Atlantic that top members of President Donald Trump’s Cabinet sent detailed operational plans and other likely highly classified information about US military strikes on Yemen to a group thread on a messaging app, to which a reporter had accidentally been added.

The Trump administration acknowledged the messages, sent over the non-government encrypted chat app Signal, seemed to be authentic without offering any explanation as to why senior officials were discussing national defence information outside of approved classified government systems.

According to The Atlantic, national security adviser Mike Waltz earlier this month convened a text conversation with top US officials, including Vice President JD Vance, Defence Secretary Pete Hegseth, and Secretary of State Marco Rubio, to discuss strikes on Houthi militants in Yemen who had been threatening international shipping in the Red Sea. Waltz, apparently accidentally, added The Atlantic editor in chief Jeffrey Goldberg to the chain.

The messages started with a discussion about when the action should be launched, while Goldberg followed along. The strikes were carried out, and the principals congratulated themselves on a job well done during a brief after-action discussion before Goldberg removed himself.

“Dear Sweet Baby Jesus,” said one former senior US official, reacting to the reporting.

Hegseth—who, according to The Atlantic, sent “operational details of forthcoming strikes on Yemen” during the course of the conversation—denied on Monday evening that war plans were discussed over text, despite the Trump administration’s earlier acknowledgement that the messages appeared authentic.

“Nobody was texting war plans, and that’s all I have to say about that,” Hegseth told reporters when asked why those details were inadvertently shared with Goldberg, after landing at Joint Base Pearl Harbor-Hickam in Hawaii.

The Defence Secretary also took jabs at the journalist, whom he described as “deceitful and highly discredited.”

Signal is an encrypted messaging app that is popular around the world, including among journalists and government officials.

Biden administration officials also routinely used it to discuss logistical planning for meetings and, at times, to communicate with foreign counterparts.

But the use of Signal to discuss planning for military operations—among the most closely guarded secrets the United States has, in part because of the potential impact on American service members’ lives—is a shocking risk to national security, current and former officials said.

Multiple officials said they could recall no instance in which Signal was used to communicate classified information or discuss military operations.

The top officials on the group chat have access to classified communications systems—including secure lines in their cars, according to one former senior official—and they have staff whose job it is to ensure communications of sensitive information remain secure.

“They broke every procedure known to man about protecting operational material before a military strike,” said a former senior intelligence official. “You have a total breakdown in security about a military operation.”

“No,” another said, flatly, when asked if there was any analogous use of the app during the Biden administration.

Even as Trump’s press secretary Karoline Leavitt said Monday that he had the “utmost confidence in his national security team, including National Security Advisor Mike Waltz,” a sense of concern was palpable inside the West Wing and across key agencies as questions emerged over whether new guidance or rules should be put in place for internal communications.

“Everyone is on Signal, all day and night,” one official told CNN, speaking on condition of anonymity to discuss internal deliberations in the West Wing. “That may well change.”

Trust in the security of Signal is bolstered by the fact that the app is open-source, meaning its code is available for independent experts to scrutinise for vulnerabilities. But like any messaging app with high-value targets, state-backed hackers have tried to find a way into Signal chats—leaving open the possibility that it may be vulnerable to prying eyes.

A report last month from Google-owned security firm Mandiant found that Russian-linked spies had tried to break into the Signal accounts of Ukrainian military personnel by posing as trusted Signal contacts.

For now, Trump has given no indication that he plans to fire anyone over the matter. He expressed surprise when asked about the story, telling reporters Monday afternoon, “I don’t know anything about it. I’m not a big fan of The Atlantic. It’s, to me, it’s a magazine that’s going out of business. I think it’s not much of a magazine. But I know nothing about it.”

But several Trump administration officials noted that not only was the move seen as a major unforced error with serious national security implications, Trump holds a personal disdain for Goldberg, adding insult to injury in the matter.

When the president was briefed Monday afternoon on The Atlantic story, he expressed his contempt for the journalist, two sources familiar with the briefing told CNN.

“You couldn’t have picked a worse person than Goldberg to add to the chat,” one person close to Trump said.

A mistake that would normally prompt an investigation

Using a Signal chat to share highly classified information and accidentally including a reporter on the discussion also raises the possibility of violations of federal laws such as the Espionage Act, which makes it a crime to mishandle national defence information. It’s a law that was used in the Justice Department’s prosecution of Trump for hoarding classified documents in unauthorised locations such as a bathroom at Mar-a-Lago after leaving his first term.

Under normal circumstances, a mistake such as this would prompt an investigation by the FBI and the Justice Department’s national security division, according to former Justice Department officials.

That’s unlikely here, in part because some of the top Trump administration officials on the Signal chat would be the ones to ask for such an investigation.

The Justice Department typically relies on receiving a crime report from the originating agency of the national defence information—in this case, the Defence Department. The senior officials in the discussion also have what’s known as original classification authority, meaning they can downgrade the classification status of the information.

But if lower-level government officials made a similar mistake, there’s little doubt there would be consequences, including possible loss of their security clearances, current and former officials say. Pentagon regulations specifically state that messaging apps, including Signal, “are NOT authorised to access, transmit, or process non-public DoD information.”

“If anyone else did it, no question it would be investigated,” a former Justice official said.

The response amongst Republicans on Capitol Hill was mixed. Sen. John Cornyn, a senior Texas Republican, called it a “huge screw up.”

“Is there any other way to describe it? I can use a different word but you get the drift,” Cornyn told CNN and other reporters.

He added: “I would hope the interagency would look at that. It sounds like somebody dropped the ball.”

But Republican House Speaker Mike Johnson downplayed the significance of the incident, “What you did see, though, I think, was top level officials doing their job, doing it well, and executing on a plan with precision.”

“Apparently an inadvertent phone number made it onto that thread. They’re going to track that down and make sure that doesn’t happen again,” he told CNN.

‘Somebody needs to get fired’

Over the course of the conversation, Hegseth sent “operational details of forthcoming strikes on Yemen, including information about targets, weapons the US would be deploying, and attack sequencing,” according to The Atlantic. Elsewhere in the conversation, CIA Director John Ratcliffe sent “information that might be interpreted as related to actual and current intelligence operations.”

All would almost certainly be classified at the highest level, former officials said.

“Somebody needs to get fired,” former Defence Secretary and CIA Director Leon Panetta told CNN. “How the name of a journalist was added to that list – this is just a serious blunder,” he said, noting that if it had been someone other than Goldberg, they “could reveal this information immediately to the Houthis in Yemen that they were about to be attacked, and they in turn could have… attacked US facilities in the Red Sea, causing casualties of our troops.”

The US government has several systems in place to transfer and communicate classified information, including the Secret Internet Protocol Router (SIPR) network and the Joint Worldwide Intelligence Communications System (JWICS). Senior government officials, including the defence secretary, vice president, secretary of state, and others, have access to these systems at virtually all times, including phones and laptops specifically configured for classified information.

A former senior US defence official said you cannot send secret information from one of these systems to an unclassified network. Hegseth—or someone working for him—would have had to do it manually. The official said this amounts to a blatant mishandling of classified information and an illegal transfer of the material from a classified system to an unclassified network.

Hegseth “somehow had to transfer it or copy it to get it onto Signal in the first place,” the official said. “You can’t forward a classified email to an unclassified system. You would either have to print it out or type it up while looking at both screens. So he had to have done it or somebody would’ve had to have done it for him that way.”

“This appears to be an authentic message chain, and we are reviewing how an inadvertent number was added to the chain. The thread is a demonstration of the deep and thoughtful policy coordination between senior officials. The ongoing success of the Houthi operation demonstrates that there were no threats to troops or national security,” Brian Hughes, National Security Council spokesperson, said in a statement to CNN.

Vance is quoted in the texts expressing concern about the strikes being a “mistake” and uncertainty that Trump was aware of how strikes on the Houthi rebels would be inconsistent with messaging on Europe.

“I am not sure the president is aware how inconsistent this is with his message on Europe right now. There’s a further risk that we see a moderate to severe spike in oil prices. I am willing to support the consensus of the team and keep these concerns to myself. But there is a strong argument for delaying this a month, doing the messaging work on why this matters, seeing where the economy is, etc.” Vance wrote in the Signal group chat, according to The Atlantic.

Trump administration officials appear to have reacted to that element of the reporting—not any suggestion that the use of Signal at all for this purpose raised national security concerns.

In a statement to CNN, William Martin, communications director to the vice president, said, “The Vice President’s first priority is always making sure that the President’s advisers are adequately briefing him on the substance of their internal deliberations. Vice President Vance unequivocally supports this administration’s foreign policy. The President and the Vice President have had subsequent conversations about this matter and are in complete agreement.”

Democrats on Capitol Hill instantly reacted with outrage, with at least one senior member signalling that he planned to press senior intelligence officials when they appear before Congress at a previously scheduled hearing before the House Intelligence Committee on national security threats on Wednesday. (The same officials, including Ratcliffe, will also appear before a Senate panel on Tuesday.)

“I am horrified by reports that our most senior national security officials, including the heads of multiple agencies, shared sensitive and almost certainly classified information via a commercial messaging application, including imminent war plans,” said Rep. Jim Himes, Connecticut, the top Democrat on the House Intelligence Committee, citing the “calamitous risks of transmitting classified information across unclassified systems.”

“If true, these actions are a brazen violation of laws and regulations that exist to protect national security, including the safety of Americans serving in harm’s way.”

Some of the participants in the text chain have in the past railed against the use of non-government platforms to conduct sensitive official business.

“Hillary Clinton put some of the highest, most sensitive intelligence information on her private server because maybe she thinks she’s above the law,” then-Florida Senator Marco Rubio said at a town hall event in Iowa in 2016. “Or maybe she just wanted the convenience of being able to read this stuff on her Blackberry. This is unacceptable. This is a disqualifier.”