GREENVILLE, S.C. - Testing showed no signs of contamination at a post office where a vial containing the deadly poison ricin was found, federal officials said Thursday.

The post office was shut down Wednesday night and workers moved to another location after federal health officials determined material in a vial found inside an envelope a week earlier was ricin.

"They've tested the whole facility...and there was no evidence of ricin in the facility at all," Postal Service spokesman Gerry McKiernan said Thursday.

Health workers took a total of 80 samples from the post office. Postal officials said the facility will reopened Friday afternoon.

FBI spokeswoman Kathleen Murphy wouldn't answer questions about possible suspects, but said the investigation was continuing. Terrorism was not suspected, but Homeland Security Department spokesman Brian Roehrkasse said the package was "related to threats criminal in nature."

William Brown, another Postal Service spokesman, said three employees came in contact with the envelope, but he did not think they were exposed to the ricin. "There was no substance on the outside of the envelope at all," Brown said. "We do not feel that there is any risk to the employees."

A spokesman for the South Carolina Department of Health and Environmental Control said his agency interviewed 25 employees and none reported symptoms or illnesses. The agency plans to talk with at least six more employees, DHEC spokesman Thom Berry said.

A letter in the envelope contained a warning that large amounts of ricin would be dumped into drinking reservoirs around the country if the federal government did not change a rule requiring truckers to rest after 10 hours on the road, said a senior law enforcement official speaking on condition of anonymity.

Officials would not say to whom the envelope was addressed or where it was postmarked. The federal law enforcement official did say the letter was not addressed to a government official.

Officials said it was unlikely the letter's author could carry through on that threat. Thousands of pounds of ricin would be necessary to contaminate water supplies, and exposure to chemicals used in water treatment would probably render the ricin harmless.

The incident was the first confirmed use of a letter with a deadly chemical or biological agent since the anthrax attacks two years ago, the official said. There have been hoaxes over that time, however.

Ricin is derived from the castor bean plant, is relatively easy to make and can be deadly in very small doses. When inhaled or ingested, fever, cough, shortness of breath, chest tightness and low blood pressure can occur within eight hours. Death can come between 36 and 72 hours after exposure. There is no antidote.

Workers in one neighboring business said they were concerned about poison being found in the mail.

"We're going to take every precaution here," said Tim Valentine, 38, of Greenville, who works at the Ted Lansing Corp. near the post office. "It does make us stay on our toes."

Erika Tillery, a 25-year-old pharmacy technician who works down the street from the main Greenville post office, said she had to ask her pharmacist what ricin was. She said she didn't think there was much cause for alarm. "I think two years ago, people would have been more freaked out."

The envelope with the vial carried the typewritten message "caution-Ricin-poison" on the outside, according to a statement by the Greenville County Sheriff's Office. It arrived at a facility between 2 a.m. and 3 a.m. on Oct. 15, the sheriff's office said.

A postal worker noticed the wording and law enforcement officials were called.

"We talk to our employees every single day about safety. They all know the procedure on suspicious packages," Brown said. "They did exactly what they're supposed to do. They did a great job on this."

The letter was sent to the Centers for Disease Control and Prevention, which determined Tuesday that the vial contained ricin.

ON THE NET

CDC: www.cdc.gov

FBI: http://www.fbi.gov/