(Reuters) - Three members of Congress have received threatening mail over the last two days and law enforcement officials Wednesday warned that more may be coming.

At least two of the letters - one to the home-state office of a senator and the other to the district office of a member of the House of Representatives - contained a suspicious powdery substance. Tests found the substance to be harmless, according to a statement by Senate Sergeant at Arms Terrance Gainer, the chamber's chief law enforcement officer.

In a statement to members of the Senate, Gainer did not say what was found in the third letter.

Gainer's notice, Urgent: Suspicious Mail Alert, warned that the unidentified sender of the letters indicated that additional letters containing a powdery substance will be arriving at more Senate offices and that some of these letters may contain actual harmful material.

He warned that special attention should be paid to letters postmarked from Portland, Ore.

(Reporting By Richard Cowan and Thomas Ferraro; Editing by Sandra Maler)