Director of National Intelligence Tulsi Gabbard and CIA Director John Ratcliffe - both of whom were in the chat - testified before the Senate Intelligence Committee that no classified material was shared in the group chat on Signal, an encrypted commercial messaging app.
But Democratic senators voiced skepticism, noting the journalist, Atlantic editor-in-chief Jeffrey Goldberg, reported Defense Secretary Pete Hegseth posted operational details about pending strikes against Yemen's Iran-aligned Houthis, "including information about targets, weapons the US would be deploying, and attack sequencing".
Committee members said they planned - and Gabbard and Ratcliffe agreed to - an audit of the exchange.
The Senate's Republican majority leader, John Thune, said he expected the Senate Armed Services Committee to look into Trump administration officials' use of Signal.
"It's hard for me to believe that targets and timing and weapons would not have been classified," Senator Angus King, a Maine independent who caucuses with the Democrats, said at the contentious hearing, which featured several sharp exchanges.
Gabbard repeatedly referred questions about the exchange to Hegseth and the Department of Defense.
She and Ratcliffe will face more lawmakers on Wednesday when the House of Representatives will hold its annual "Worldwide Threats" hearing. Democrats said they planned to discuss the Signal chat.
The revelation on Monday drew outrage and disbelief among national security experts and prompted Democrats - and some of President Donald Trump's fellow Republicans - to call for an investigation of what they called a major security breach.
"I am of the view that there ought to be resignations, starting with the national security adviser and the secretary of defense," Democratic Senator Ron Wyden of Oregon said at the hearing.
But Trump voiced support for his national security team when questioned about the incident at a White House event on Tuesday with Michael Waltz, his national security adviser, who mistakenly added Goldberg to the Signal discussion.
Trump said the administration would look into the use of Signal. He said he did not think Waltz should apologise, but said he did not think Waltz and the team would be using Signal again soon.
Waltz, in an interview on Fox News, said, "I take full responsibility" for the breach, as he had created the Signal group, but he emphasised there was no classified information shared.
Waltz said the situation was "embarrassing" and that the administration would "get to the bottom" of what went wrong. He said Goldberg's number was not saved in his phone and he does not know how the journalist was mistakenly added to the chat group.
Sensitive information is not supposed to be shared on commercial mobile phone apps. Additionally, Signal's ability to erase conversations would violate laws governing the retention of government records.
Accounts appearing to represent Vice President JD Vance, Secretary of State Marco Rubio, Hegseth, Ratcliffe, Gabbard, Treasury Secretary Scott Bessent, White House chief of staff Susie Wiles, and senior National Security Council officials were assembled in the chat group, Goldberg wrote on Monday.
Gabbard acknowledged she had been abroad during the chat, although she declined to say whether she was using a private phone.
The White House sought to play down the incident. White House press secretary Karoline Leavitt accused Goldberg of sensationalising the story in a post on X.
Also on X, White House communications director Steven Cheung dismissed as "faux outrage" the concern over the inclusion of a journalist in a war-planning chat.
Hegseth told reporters on Monday that no one had texted war plans. Goldberg, appearing on CNN on Monday, called those comments "a lie".
It remained unclear why the officials chose to chat via Signal rather than the secure government channels typically used for sensitive discussions.
