Here is more from the Washington Post:
In pre-plea proceedings in federal court in Washington, Olson’s defense counsel said he admitted to the misdemeanor charges — each punishable by up to one year in prison at his sentencing Sept. 13 — and cooperated with federal prosecutors on the understanding that they were also investigating and pursuing criminal charges against retired four-star Marine Gen. John G. Allen. The latter commanded U.S. and NATO forces in Afghanistan before retiring in 2013. He was tapped in late 2017 as president of Washington’s influential Brookings Institution, to which Qatar agreed in 2013 to donate $14.8 million over four years.
Allen reportedly has denied ever working as a Qatari agent and said his efforts on Qatar in 2017 were motivated to prevent a war from breaking out in the Gulf that would put US troops at risk. Allen has "voluntarily cooperated" with the government's investigation.
Update: It was reported on June 8 that the FBI has seized Allen's data related to Qatar lobbying.
Here is more from the New York Times of Brookings' affiliation with Qatar:
Brookings once had a large campus in Doha. The campus was established years before General Allen became president of the think tank. In September, according to its website, Brookings ended its affiliation with the Doha institute, which is now called the Middle East Council on Global Affairs.
Allen became president of the think tank in November of 2017.
Update II: John Allen has been placed on administrative leave, and EVP Ted Gayer is now serving as acting president of Brookings.
Last week it was announced that Gayer is leaving Brookings to head the Niskanen Center, effective August 2022.