Adam Hickey, a deputy assistant attorney general, told Senate lawmakers that the Foreign Agents Registration Act — a law aimed at ensuring transparency about lobbying efforts done in the U.S. at the direction of foreign governments or principals — contains multiple exemptions for registration and requires proof that someone intended to break the law by failing to disclose their work.

He said lawyers in a specialized Justice Department unit often prod someone to voluntarily register instead of seeking to charge them.

"The high burden of proving willfulness, difficulties in proving direction or control by a foreign principal and exemptions available under the statute make criminal prosecution for FARA violations challenging," Hickey said.

Nonetheless, he said, the Justice Department will not refuse to bring a case if there is evidence of intentional wrongdoing. It has brought four criminal prosecutions under the statute since 2007, all of which he said have resulted in convictions.