Police Lt. Lewis Roberts said investigators don't believe foul play was involved, and the FBI said the case didn't appear to be terrorism-related but federal authorities were assisting local police.
The comatose man was believed to have been living in the room at the Extended Stay America Motel several blocks west of the Las Vegas Strip where a vial of powder identified as ricin was found Thursday afternoon.
Officials said the vial was in a plastic bag, and castor beans also were found in the room.
The man's identity was not made public.
"He's in very critical condition," Roberts said.
Police said three motel employees and three police officers were also hospitalized, mostly as a precaution.
Ricin is made from processing castor beans, and can be extremely lethal. As little as 500 micrograms, or about the size of the head of a pin, can kill a human, according to the Centers for Disease Control and Prevention in Atlanta.
Police were called to the motel Thursday and retrieved a package from the motel manager that was determined to be a chemical or controlled substance, Officer Ramon Denby said.
A 19-member Nevada National Guard team specializing in weapons of mass destruction took samples of the material from the motel office and a police evidence locker and initially identified it as ricin, said Air National Guard Capt. April Conway, spokeswoman for the state National Guard office in Carson City.
Conway said Friday the National Guard team was no longer involved in the police investigation.
Denby said two preliminary tests indicated the substance contained ricin, and that results from further tests by the Centers for Disease Control and Prevention and a second local lab were expected Friday.
A man who brought the substance to the manager told police he found it in a suite and it did not belong to him, police Capt. Joseph Lombardo told a news conference Thursday night. Lombardo did not identify the man.
"Ricin has no medical uses other than cancer research," Lombardo said. "An individual citizen other than being involved in cancer research or cancer prevention would not have any legal means or proper means of having that."
Police cordoned off the area and isolated the room where the substance was found.
Three motel employees and another person were quarantined and decontaminated at the site, then taken to hospitals for further testing, Denby said. All appeared to be in good condition, he said. Three police officers who had been exposed were also taken to hospitals.
Someone exposed to ricin shows signs of contamination in six to eight hours, Denby said.
Homeland Security officials joined the FBI and Southern Nevada Health District in the investigation.
A woman who answered the phone at the Extended Stay America Motel declined to comment.
