A chemical odor at a Silicon Valley hotel caused 12 people to be taken to hospital, but a hazardous materials team could not determine the source of the smell, authorities said on Friday.

The odor was first noticed on Thursday night at a Westin hotel in Palo Alto, south of San Francisco, and paramedics responded to the scene, the city's fire department said in a statement.

Twelve people were "decontaminated" at the hotel and later sent to two area hospitals after multiple complaints of a respiratory irritant from employees and guests at the hotel, the department said.

Authorities ordered the evacuation of the garage and the first floor of the hotel. A hazardous materials team found swimming pool chemicals at the hotel but their containers were intact, and the team could not identify the source of the odor, according to the fire department.

Guests were allowed back into the evacuated parts of the hotel just after midnight, a fire department statement said.

The Westin said in a statement, "We are pleased that all guests and associates who were initially taken to the hospital to be checked have been released."

An investigation of the cause of the odor will continue, the hotel statement said.

(Reporting by Alex Dobuzinskis in Los Angeles; Editing by Daniel Wallis and Dan Grebler)