WASHINGTON -- The FBI has searched the apartment of former Reagan administration national security adviser Robert McFarlane for evidence of whether he lobbied on behalf of the government of Sudan in violation of federal law.

A search warrant on file in U.S. District Court in Washington shows agents seized items this month including handwritten notes about Sudan and White House documents with classification markings up to Top Secret.

It is against the law for Americans to do business with Sudan because of its alleged support for international terrorism and human rights violations, among other things. Sudan's president, Omar al-Bashir, has been charged by the International Criminal Court with genocide and other crimes during the deadly conflict in Sudan's western Darfur region.

In an accompanying affidavit, FBI agent Grayden R. Ridd cited a host of emails between McFarlane and Sudanese government officials obtained prior to the search.

"I believe that these emails are evidence that McFarlane was entering into an agreement with the government of Sudan to lobby the U.S. government officials on behalf of Sudan and to provide it advice during negotiations with the United States," Ridd wrote. He said he believed the emails are also evidence of an attempt by McFarlane and a Sudanese government official "to hide McFarlane's relationship with Sudan by construing the agreement to make it appear that his contractual relationship was with Qatar, when in fact it was not."

The affidavit said that the FBI investigation has established that in February 2009, McFarlane entered into a one-year agreement with the government of Sudan to act as its consultant and to lobby the U.S. government on its behalf.

Ridd wrote that the source of the emails to McFarlane appeared to be someone from the Sudanese intelligence service.

The affidavit is listed as "under seal" but is viewable online.

The FBI is also investigating whether McFarlane violated a law that requires anyone working as a foreign agent of another country to disclose that to the Foreign Agent Registration Act Unit of the Justice Department.

The investigation into McFarlane was first reported by The Washington Post.

McFarlane has not been charged with a crime. The case is being handled by the U.S. attorney's office for the Eastern District of Virginia. A spokesman for the office, Peter Carr, said McFarlane is cooperating with the ongoing investigation and, through his counsel, has asserted his innocence.

McFarlane's lawyer, Barry Levine, did not immediately return telephone and email messages Thursday. Levine told The Post that McFarlane didn't violate any laws.

"He has devoted his entire adult life to the interests of this country, and he cares deeply about the people of Darfur," Levine told the newspaper.