COLUMBIA, S.C. (AP) _ A South Carolina postal facility was closed and tested for the poison ricin after a vial was found inside an envelope, but FBI officials said Thursday there were no signs of contamination.

The vial found at the postal facility in Greenville last week tested positive for ricin, federal officials said Wednesday.

Terrorism was not suspected but Homeland Security Department spokesman Brian Roehrkasse said the package was ``related to threats criminal in nature.''

FBI spokeswoman Kathleen Murphy would not answer questions about possible suspects, but said the investigation was continuing Thursday.

The envelope carried the typewritten message ``caution-Ricin-poison'' on the outside, according to a statement by the Greenville County Sheriff's Office.

The letter contained a warning that large amounts of ricin would be dumped into drinking reservoirs 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 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.

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.

The incident was the first confirmed use of a letter containing 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.

William Brown, spokesman for the U.S. Postal Service, 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.''

As a precaution, workers were moved to another Greenville facility while tests were conducted on the building where the envelope was received.

Murphy did not say Thursday whether testing on the postal facility had concluded. A South Carolina post office spokesman did not immediately return a call to determine when the facility might reopen.

The envelope arrived at the Greenville 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.

The worst bioterrorism attack in U.S. history was perpetrated through the mail two years ago. Five people died and 17 were sickened by anthrax-infected letters sent to media companies and the Capitol Hill offices of Democratic Sens. Tom Daschle of South Dakota and Patrick Leahy of Vermont.

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.

The FBI repeatedly has warned local police about the possibility that terrorists might use ricin in an attempt to poison people through ventilation systems, drinking supplies or in food.

British police earlier this year arrested seven members of an Algerian extremist group on charges of plotting to use ricin to kill a small number of people and terrify the London population. Instructions for making ricin also were found in an al-Qaida safehouse in Kabul, Afghanistan, according to the FBI.

Ricin also has been used in crimes in the United States that have no connection to terrorism. Last summer, a Washington state man was convicted of making and possessing about 3 grams of ricin, enough to kill 900 people.

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Associated Press Writer Curt Anderson contributed to this report from Washington.

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On the Net:

CDC: www.cdc.gov

FBI: http://www.fbi.gov