It is not every day that two prominent lawyers are brought before a federal grand jury and directed to provide documents and testimony about conversations they had with a wealthy client.

But that is what happened with two partners at Williams & Connolly, the prestigious Washington law firm, who are representing Morris E. Zukerman, a former Morgan Stanley banker and oil investor. Last month, Mr. Zukerman was accused of failing to pay $45 million in income and sales taxes on works of art and profits from the sale of an oil company.

A series of court filings in Mr. Zukerman’s pending criminal case in Federal District Court in Manhattan shines a light on the often-unseen role lawyers can play in nonpublic tax investigations by the Internal Revenue Service and federal prosecutors. In the filings, federal prosecutors in Manhattan raised the prospect of potential conflict of interest for the two lawyers, who are trying to negotiate a plea deal for Mr. Zukerman.

Typically, lawyers cannot be compelled to testify or produce evidence against a client in a grand jury investigation. But in rare cases, judges can require it, if there is evidence that clients’ communication with their lawyers was done purposely to further a crime or a fraud. In the law, it is known as the crime-fraud exception to the attorney-client privilege.