A section of the Las Vegas airport was closed for several hours after a flight arrived from Denver with 11 sick people on board, officials said Sunday.

Passengers and crew members on United Airlines Flight 1491 were quarantined for several hours Saturday night at McCarran International Airport while the plane, passengers and luggage were checked by hazardous materials experts, said Elaine Sanchez, a McCarran spokeswoman.

Four of the victims were flight attendants, Sanchez said.

Sanchez said the cause was still being investigated. A passenger, Don Yarbrough, told KUSA TV of Denver that investigators believed the victims reacted to a cleaning fluid containing ammonia that had been used on the plane after it arrived in Denver from Cancun, Mexico.

A United spokeswoman in Chicago, Robin Urbanski, said victims felt weak and nauseous but she had no other details.

There were 143 passengers and crew on the plane when it landed in Las Vegas.