"We did have one sample that had trace detectible levels of ricinine," said Pat Armour, manager of the Southern Nevada Public Health Laboratory.
The motel room's occupant remains in a hospital after police said they found vials of ricin, a poison that can be lethal in minuscule amounts, along with castor beans.
Officials have said since ricin was discovered Feb. 28 that they had not found evidence in the motel room or elsewhere of ricin contamination. The substance for which the police officer tested positive is an alkaloid extracted from the seeds of the castor plant and does not derive directly from ricin.
It was not immediately clear Thursday what effect that could have on the investigation into where the ricin came from. Las Vegas police declined to comment on the investigation.
KTNV-TV in Las Vegas identified the officer as Jim Mitchell and aired an interview with his wife, Regina Mitchell, who said she was concerned about the test result. Police would not confirm the officer's identity to the AP.
About 5 percent of the U.S. population is believed to have similar trace levels of ricinine in their system, Armour said. It's not dangerous at that level, and it's possible that the officer's urine test results stemmed from exposure to castor oil, cosmetics, particle board, paints or other products derived from castor beans, Armour said.
An FBI spokesman said the officer was the only person involved in the case to have tested positive for ricinine.
Officials have said they could not conclusively say the man who lived in the room, Roger Bergendorff, was poisoned. The poison wasn't discovered in his room until two weeks after his hospitalization.
"Beyond the possibility of Mr. Bergendorff, we are not aware of anyone else who has become ill as a result of ricin exposure from this matter," said agent Joseph Dickey, a spokesman for the FBI office in Las Vegas.
Bergendorff, 57, an unemployed graphic artist, summoned an ambulance Feb. 14, complaining of respiratory distress. He spent almost four weeks in a coma and has been treated for kidney failure. He remained in fair condition Thursday at a Las Vegas hospital.
Bergendorff told his younger brother he believed he had been exposed to the ricin powder. But he also told family members he had no intention of hurting anyone, and that he had the deadly poison for "self-defense."
Authorities have refused to say whether they plan to charge Bergendorff with state or federal crimes.
Ricin is categorized as a biological agent under a federal law that provides for the possibility of life in prison and fines for production, acquisition or possession of it.