Special counsel John Durham is reportedly scrutinizing British ex-spy Christopher Steele’s main source for his discredited dossier, using a subpoena to obtain documents from the Brookings Institution related to its employment of Igor Danchenko, a researcher who visited Russia in 2016 as he worked for the ex-MI6 agent.

The New York Times reported on Monday, citing “people familiar with the investigation,” that Durham “has keyed in on the FBI’s handling of a notorious dossier of political opposition research both before and after the bureau started using it to obtain court permission to wiretap a former Trump campaign adviser.” The outlet said Durham obtained records from the left-leaning think tank in Washington, D.C., related to Danchenko, who worked for the organization from 2005 to 2010 as a Russia researcher but is best known as the main source for Steele’s discredited Trump-Russia dossier in 2016.