WASHINGTON — Criminal prosecutions are rare for people who fail to register as foreign agents, according to a top Justice Department official who testified Wednesday about an obscure law receiving new attention amid investigations into contacts between the Donald Trump campaign and Russia.

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 law has been broadly discussed in the last year because of Justice Department investigations into Trump campaign associates and because of a watchdog report last year that said the statute had been weakly enforced for decades. The report made several recommendations for improvement.