GREENVILLE, S.C. (AP) - Officials at the national Centers for Disease Control and Prevention have begun a daily watch for ricin, a month after authorities discovered the deadly toxin at Greenville's airport mail center.

The Atlanta-based health agency is studying data reports from the nation's poison control centers. The Oct. 15 Greenville incident was the first time the poison was found in a public facility in the United States, said Dr. Martin Belson, a CDC toxicologist.

In the two weeks after the poison was found, CDC officials reviewed 97,000 poison calls nationwide, looking for anything unusual that might be an outbreak of the toxin. However, officials didn't find it, according to a report issued by the CDC Thursday.

Officials initially investigated two South Carolina cases in which there was "multi-organ failure," which can occur in ricin poisoning, but both cases turned out to be caused by other illness, Belson said.

Health departments and emergency rooms also were told to watch for possible ricin-related illnesses, once the CDC confirmed the substance was ricin on Oct. 21, he said.

The CDC and state health officials examined each of the mail facility's 36 postal workers between Oct. 21 and Oct. 23, Belson said, and found no illness that could be linked to ricin.

The federal agency also reported no contamination after taking more than 70 samples from inside the mail center. The mail facility was closed Oct. 22, but reopened on Oct. 24.

The report did not discuss the weeklong delay in notifying the public about the ricin or the five days it took for state officials to send samples to the CDC for testing.

State health officials say the package was not sent to the CDC any sooner because they were convinced the package had not been breached, was not exposed to anyone and there were no reports of any illness that might have been tied to the toxin.

"We did exactly what we should have done," said Jim Beasley, a spokesman for the state Department of Health and Environmental Control.

Belson said the CDC report was created to help inform public health and laboratory officials about ricin. He said the agency is watching poison control reports because the suspect is at large and the toxin could be used.

"Obviously the investigation is not ended so we can't assume this won't occur again," Belson said.

No one has been arrested in the incident, which is being investigated by the FBI. Authorities have said they believe the package was meant as an extortion attempt, even though the state's Joint Terrorism Task Force, which is made up of FBI and State Law Enforcement Division agents, investigated it. A note inside the package threatened to use the poison unless a federal regulation requiring rest for truckers was repealed.

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.