Trump admin's shifting explanations for how a journalist was added to Signal chat

How did a journalist get added to a Signal group chat among top Trump officials discussing a U.S. military strike on Houthi rebels in Yemen?

That's a key question after the message chain was brought to light on Monday by The Atlantic's Jeffrey Goldberg in an article titled: "The Trump Administration Accidentally Texted Me Its War Plans." A National Security Council spokesman told ABC News shortly after the article was published the message thread that was reported "appears to be authentic."

Goldberg said he received a connection request on the commercially available app from a user identified as White House national security adviser Michael Waltz. He recounted later being added to a chat group with a message from Waltz about it being established to coordinate about Houthis.

After the article prompted a firestorm, Goldberg's credibility was attacked by White House officials, apparently attempting to minimize what was being called by Democrats, experts and even some Republican lawmakers, a major security breach. Trump on Tuesday told reporters there was "no classified information as I understand it."

On Wednesday, Goldberg published a second story showing the purported messages. The initial story in The Atlantic had only described the operational part of the message chain, but did not divulge specifics.

A screenshot from the chain published Wednesday, from Goldberg's perspective, states: "Michael Waltz added you to the group."

The Trump administration, though, has offered different -- and at times contradictory -- explanations for how Goldberg got included.

President Donald Trump, in multiple interviews, suggested it was a staffer on Waltz's team who added him. Trump has repeatedly expressed support and confidence in Waltz amid Democratic pressure for him to resign or be fired.

In a new defense on Wednesday, Trump suggested maybe it was a "bad signal" without elaborating on what that means.

"But somebody in my group either screwed up or it's a bad signal," Trump said on "The Vince Show" with Vince Coglianese. "You know, it's a bad signal. Happens too. But seems to be maybe came in with a staffer, and it was by accident."

"For what we can tell, we'll know pretty much today, I think, but we have some bigger guys checking out the phones, but it's something that is not a big deal, other than you want to find out who did it and how they did it because you don't want to happen, you know, in the future; you can't have that happen," Trump added.

On Tuesday night, however, Waltz said he was the one responsible for what took place.

"I take full responsibility. I built the group," Waltz told Fox News' Laura Ingraham.

At the same time, Waltz tried to flip the script and blame Goldberg -- suggesting the editor could have somehow wormed his way in to the group chat.

"Of course, I didn't see this loser in the group. It looked like someone else. Now, whether he did it deliberately or it happened in some other technical mean, is something we're trying to figure out," Waltz said, without evidence, on Fox News.

Signal registers users through their phone numbers and group chats are controlled by users designated as administrators, who have the authority to invite and reject users at will.

"I didn't hack into anyone's phone," Goldberg told ABC News Live anchor Kyra Phillips on Wednesday. "Mike Waltz invited me to Signal and then he invited me to a group. I don't know how to say it more simply than that."

Director of National Intelligence Tulsi Gabbard, in testimony before the House Intelligence Committee on Wednesday, told lawmakers that adding Goldberg was a "mistake."

"It was a mistake that a reporter was inadvertently added to a signal chat with high-level national security principles, having a policy discussion about imminent strikes against the Houthis and the effects of the strike," Gabbard said.

White House press secretary Karoline Leavitt on Wednesday said Waltz "has taken responsibility for this matter" and it's under review.

"The National Security Council immediately said alongside the White House counsel's office that they are looking into how a reporter's number was inadvertently added to this messaging thread," Leavitt added.

That review, Leavitt said, also includes Elon Musk's team.

"Elon Musk has offered to put his technical experts on this to figure out how this number was inadvertently added to the chat. Again, to take responsibility and ensure this can never happen again," she said.

ABC News' Ivan Pereira contributed to this report.