The letter rails against the Metropolitan Square development in downtown Washington for being too tall. Along with the letter is a Senate bill that would designate a substantial portion of the city for an historic trust.

Both the letter, purportedly signed by the executive director of the National Capital Planning Commission, and the would-be legislation look strikingly real. But according to commission officials, both are demonstrably fake.

The Justice Department has been asked to investigate the mysterious mailing of copies recently to area planning and civic groups.

"None of it has derived from our commission," said NCPC spokesman Robert Gresham, adding that officials first tried to ignore the bogus material but "some people were giving it some credence and were spending some time on it."

Gresham said the commission knows of at least 20 groups that have received copies of the letter and the phony bill, one of them Prince George's County. All have an interest in planning and historic preservation, he said, but the motives behind the literature remain vague.

The letter, written over the name of commission Executive Director Reginald W. Griffith, complains about Metropolitan Square as a "security threat to the White House" because of its proposed height.

Gresham said the commission has never taken a position on the building project.

The printed bill establishing a new trust area contains a number of planning measures that have been proposed in Congress at various times, Gresham said, but never presented as a single piece of legislation.

Gresham said attorneys for the commission hope to meet with lawyers at Justice this week to discuss the incident, but that it is unclear whether any laws may have been broken.