Two college students were arrested for an anthrax hoax that halted postal service in the town of Murray after white powder spilled from an envelope.

Preliminary tests indicated the substance was powdered sugar, Murray Postmaster Mark Kennedy said Wednesday.

The post office in the southwestern Kentucky community was closed Tuesday after the powder spilled onto a postal clerk's shirt.

Amy Wood, 20, of Benton, Ky., and Erin Creighton, 21, of Morganfield, Ky., both students at Murray State University, were arrested Tuesday. They allegedly intended to send the letter to friends as a hoax, Murray police Capt. Eddie Rollins said.

"I don't think they understood the seriousness of their hoax initially, but I think they were coming to understand the seriousness of it when they were taken into custody," Rollins said.

Both were charged with a single count of mailing a threatening communication, a felony punishable by up to five years in prison, said U.S. Attorney Stephen Pence.

Three postal employees underwent medical tests as a precaution, Kennedy said.

Elsewhere, six people in the Philadelphia area were arrested on charges they perpetrated anthrax hoaxes, including one that shut down a Home Depot store in Chester County for five days, First Assistant U.S. Attorney H. Geoffrey Moulton Jr. said Wednesday.

Robert T. Gibson of West Chester, Pa., is accused of sending an envelope containing a white powder and a note claiming the substance was anthrax to the Home Depot in Frazer. The powder tested negative for anthrax and the store reopened Tuesday.

Charges were filed Wednesday in Flint, Mich., against three General Motors Corp. workers accused of taking part in an anthrax hoax that shut down a production line for hours, Genesee County Prosecutor Arthur A. Busch said. An envelope containing baby powder was placed on a truck on the assembly line, authorities said.

In western Tennessee, a former Northwest Airlink employee was indicted Tuesday on charges he falsely reported he had been kidnapped and forced to phone a terrorist threat to Northwest Airlines. If convicted, Timothy Scott McNeill, 40, of Memphis, could get five years in jail.