LAS VEGAS -- Metro Homeland Security reports four people were sent to the hospital for possible ricin poisoning at the Extended-Stay America hotel at West Flamingo Road and South Valley View Boulevard.

At about 3 p.m. Thursday, an employee found a substance in one of the hotel's apartments when he was cleaning it and took it to the office. The four people, three of whom were employees, who were exposed to the substance were sent to the hospital, and the area surrounding the apartment was evacuated, according to Metro.

As of Thursday night, the four people were OK.

Preliminary reports indicate the substance may have been ricin, a substance that is 6,000 times more toxic than cyanide. There is no known antidote.

Ricin is used in cancer research. It is made from crushed castor beans with liquid added to them, and if inhaled or ingested, a fatal dose is 0.2 mg, or the size of a pinhead.

Metro said there was no indication anyone was making ricin, but they are looking for the people who last stayed in the apartment. They will not elaborate on the amount found.

Officials said this is not considered to be a terrorist threat.

The area has been contained, and the hotel has been quarantined for an indefinite amount of time.