The two men had known each other for years around Capitol Hill. Both were hawkish supporters of Israel and active in the Republican Party. But now they were on opposite sides of a remote dispute between wealthy Persian Gulf monarchs: One was a lobbyist for Qatar, and the other had sided strongly with rivals who sought to blockade the tiny emirate.
“Be very careful,” the lobbyist, Nicolas D. Muzin, warned on March 5, in a conversation quoted Thursday in a legal complaint accusing Qatar of hacking into the computers of its American opponents.
Anyone Mr. Muzin pointed to was “in danger,” he told his old acquaintance, Joel Mowbray, who appears to have recorded their conversation about the Qataris. “Honestly, I know they are after you and after Broidy,” the lobbyist continued, referring to Elliot Broidy, a top Republican donor, critic of Qatar and Mr. Mowbray’s associate.
Their conversation occurred just days after The New York Times and other news organizations began publishing articles based in part on leaked emails from Mr. Broidy’s account.
Many showed him trying to exploit his access to the White House to obtain lucrative contracts with foreign governments for his private security company. Mr. Broidy’s efforts included promoting accusations that Qatar was destabilizing the region while he was landing a contract for hundreds of millions of dollars with its rival, the United Arab Emirates.
All the emails were provided to the news organizations by anonymous sources and they began a run of bad publicity for Mr. Broidy. An unrelated legal proceeding revealed a few weeks later that he had worked through the president’s personal lawyer, Michael D. Cohen, to pay $1.6 million to a former Playboy model who became pregnant during an affair.
The F.B.I. has opened a criminal investigation into allegations of hacking Mr. Broidy’s account, according to people involved.
In a lawsuit filed in a federal court in California, lawyers for Mr. Broidy have argued that the government of Qatar directed the hacking of his emails to punish him for his vocal criticism of the country. An amended complaint filed on Thursday contends that, as a lobbyist for Qatar, Mr. Muzin’s comments amounted to an admission of its guilt.
“Mr. Muzin has admitted culpability,” the complaint charges, and “as its registered foreign agent, Mr. Muzin’s admission also binds the defendant State of Qatar.”
Mr. Muzin did not respond a request for comment through his firm, Stonington Strategies, but has previously called Mr. Broidy’s allegations “specious” and “hollow.”
Qatar has denied any role in the hacking.
“His claims are completely fabricated and without merit,” Jassim al-Thani, a spokesman for Qatar in Washington, said in a statement. “It was Mr. Broidy who conspired in the shadows against Qatar — not the other way around.”
Governments around the world are increasingly capitalizing on hard-to-trace hacking to damage or embarrass their rivals, and Mr. Broidy’s lawsuit and the parallel criminal investigation are rare attempts to use the American court system to try to hold a suspected state accountable.
Another well-known antagonist of Qatar in Washington, the Emirati ambassador, has also endured extensive critical news coverage based on hacked emails leaked to The New York Times and other news organizations.
But the lawsuit’s reliance on the conversation between Mr. Muzin and Mr. Mowbray underscores the difficulty of proving responsibility for such attacks without using espionage to record an admission. That challenge evidently led Mr. Broidy and his associates to resort to their own surreptitious recording.
Two people involved in the case said on condition of anonymity that the conversation had been recorded; a lawyer for Mr. Broidy declined to comment on how it was obtained. Mr. Mowbray did not return a call seeking comment.
The amended complaint on Thursday also accused two others identified as former intelligence agents for the United States and Britain of conspiring with Qatar to carry out the hacking.
Lawyers for Mr. Broidy describe Kevin Chalker, founder of the New York-based firm Global Risk Advisors, as “a former C.I.A. cyberoperative” and David Mark Powell as “a former British intelligence operative who runs G.R.A. operations out of Qatar.”
The complaint says that Mr. Powell opened the firm’s Qatar office in October 2017, “just weeks prior to the commencement of the attack on the plaintiff,” but offers no other details or evidence of a role. Mr. Chalker and Mr. Powell did not respond to requests for comment relayed through their firm.
The fragments of conversation with Mr. Muzin appear to be the strongest evidence disclosed in the complaint that connects Qatar with the hacking.
Mr. Muzin told his old acquaintance that “Broidy’s name comes up in embassy meetings often” with Qatari diplomats, the complaint states. “I definitely identified him as somebody who was not, didn’t like them too much,” Mr. Muzin said, and “they knew he had been influential” with the White House.
Speaking after the first articles based on the leaks had appeared, Mr. Muzin knew in advance that “there is more stuff coming,” the complaint says.
But the complaint does not specify the source of his information, and adds that Mr. Muzin said his knowledge did not make him responsible.
“I did not cause the Broidy stuff, just because I have information,” Mr. Muzin reportedly said of the leaks. “I don’t know all the details, but I know that I am hearing repeatedly that there’s a lot more coming.”
Adam Goldman contributed reporting from Washington.