Federal agents escorted at least four traders from the floors of the New York Commodities Exchange and the New York Mercantile Exchange Thursday and subpoenaed others as part of a criminal investigation of the markets, officials said.

The U.S. Postal Service confirmed it was executing a criminal search warrant at an office in a World Trade Center building where the commodities exchange is based.John Slavinski, head of the New York office of the U.S. Postal Inspection Service, said inspectors were executing two search warrants for documents at the Comex and the Merc.

"A criminal investigation is underway, and I'm not allowed to comment further on it," he said.

The raid came during a federal investigation of alleged widespread illegal trading activity at Chicago's two futures exchanges, which are the nation's largest.

Market sources who asked not to be named said three silver traders were taken from the Comex pits while at least one trader was taken from the floor of the New York Merc.

Comex spokesman Robert McGrath said subpoenas were being issued to an unspecified number of commodities and oil traders who belong to the Commodities Exchange Center, where the five New York futures exchanges are based.

"We understand that per an order of investigation by the Commodity Futures Trading Commission . . . that a number of subpoenas are being issued to CEC members," he said.

McGrath said some of the subpoenas were being delivered to Comex members but did not say how many.