The FBI has deepened its investigation into retired Marine Corps Gen. John Allen on allegations that he illegally worked as a foreign agent on behalf of Qatar.

The Associated Press reported Tuesday that FBI agents filed a search warrant for Allen’s electronic communications after alleging that he made false statements to agents and withheld “incriminating” documents about his role in an illegal foreign lobbying campaign on Qatar's behalf several years ago.

“There is substantial evidence that these FARA violations were willful,” FBI Special Agent Babak Adib wrote in a search warrant application, referring to the Foreign Agents Registration Act (FARA).

Allen, who served in the Marine Corps for close to four decades and as a diplomat in the Obama administration, currently works as the head of the Brookings Institution think tank in Washington, D.C.

The investigation into Allen appears related to two other probes into illegal Qatari lobbying in the U.S.

One was into the work of political operative Imaad Zubedi, who is now serving a 12-year prison sentence on corruption charges. The other case is that of Richard Olson, a retired U.S. ambassador to the United Arab Emirates, who pleaded guilty to illegal lobbying charges last week for work he did on Qatar’s behalf.

Skyline of the Qatari capital Doha
Skyline of the Qatari capital Doha AFP / GIUSEPPE CACACE

Like Olson’s case, the investigation centers around actions he took to assist Qatar as it came under a blockade by its neighbors in June 2017. The effort was led by Saudi Arabia and the UAE, who accused Qatar of enabling terrorism and of being too close to their regional foe Iran.

Former President Donald Trump initially sided against Qatar, but during this time Allen worked to influence U.S. policy away from outright isolating the small kingdom. Specifically, the FBI said Allen emailed Trump’s then-National Security Advisor H.R. McMaster to tone down the president’s stance against Qatar.

According to the FBI, two days later then-Secretary of State Rex Tillerson issued a statement where he urged other Gulf countries to “ease the blockade against Qatar” and asked “that there be no further escalation by the parties in the region," in line with Allen's request.

Allen traveled together with Olson to visit Qatar at one point where they allegedly met with the country's ruling emir and his top officials. There Allen advised the Qataris on how to influence U.S. policy in their favor, including through covert means that could include illegal acts.

Documents also show that Allen’s first-class ticket to Qatar was covered by Zuberi, the disgraced operative. In correspondence with Zuberi, Allen was alleged to have suggested in an email that he be paid a $20,000 “speaker’s fee” for the weekend trip though no speech was given. It was also revealed that Allen had attempted to pursue what the FBI described as a “multimillion-dollar business deal” with Qatar on behalf of a company where he served on the board.

Allen has denied any wrongdoing through a statement from his spokesperson to the New York Times.

“John Allen voluntarily cooperated with the government’s investigation into this matter. John Allen’s efforts with regard to Qatar in 2017 were to protect the interests of the United States and the military personnel stationed in Qatar. John Allen received no fee for his efforts," said Allen spokesperson Beau Phillips.