The president of the Brookings Institution resigned Sunday amid a federal investigation into whether he illegally lobbied on behalf of the wealthy Persian Gulf nation of Qatar.

Retired Gen. John Allen wrote in a letter to the think tank that he was leaving with a “heavy heart” but did not offer a direct explanation.

“I know it is best for all concerned in this moment,” Allen’s letter said.

A retired four-star Marine general who led U.S. and NATO forces in Afghanistan, Allen’s announcement came less than a week after The Associated Press was first to report on new court filings that showed the FBI had seized Allen’s electronic data as part of the lobbying probe.

Allen has not been charged with any crimes and, through a spokesman, has denied any wrongdoing.

Brookings, which had put Allen on administrative leave the day after the AP’s initial report, issued a statement thanking Allen for guiding the think tank through the coronavirus pandemic and other contributions. The institution said information about the search for a new president would be forthcoming.

The new court filings detail Allen’s behind-the scenes efforts to help Qatar influence U.S. policy in June 2017 when a diplomatic crisis erupted between the gas-rich Persian Gulf monarchy and its neighbors.

An FBI agent said in an affidavit in support of a search warrant there was “substantial evidence” that Allen had knowingly broken a foreign lobbying law, and had made false statements and withheld “incriminating” documents.

Allen’s alleged lobbying work involved traveling to Qatar and meeting with the country’s top officials to offer them advice on how to influence U.S. policy, as well as promoting Qatar’s point of view to top White House officials and members of Congress, the FBI’s affidavit says.

The federal investigation involving Allen has already ensnared Richard G. Olson, a former ambassador to the United Arab Emirates and Pakistan who pleaded guilty to federal charges earlier this month, and Imaad Zuberi, a prolific political donor now serving a 12-year prison sentence on corruption charges. Several members of Congress have also been interviewed.

Brookings, one of the most prestigious think thanks in the U.S., had initially hired Allen as a senior fellow before tapping him as president in late 2017 and paying him more than $1 million a year, according to recent tax records.

“The integrity and objectivity of Brookings’s scholarship constitute the institution’s principal assets, and Brookings seeks to maintain high ethical standards in all its operations,” the think tank said in its statement Sunday.

Qatar has long been a major financial backer of Brookings, though Allen decided in 2019 to stop taking any new donations from the country.

Qatari officials have not responded to requests for comment about the Allen investigation.