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Arizona Daily Star, Mamta Popat, Associated Press
A blanket of clouds wrap around Sentinel Peak and cover Tucson, Ariz., as seen from the top of Tumamoc Hill west of the city on Wednesday, Jan. 14, 2015. The National Weather Service in Tucson says the area sees fog like this only once every couple of years. Tucson was hit hardest, but the fog extended from Santa Cruz and Cochise counties all the way north to the Phoenix suburb of Chandler.

TUCSON, Ariz. — Arizona commuters from the southern part of the state all the way to a Phoenix suburb drove through an unusual amount of fog Wednesday morning.

The fog may have contributed to a fatal accident in Casa Grande, where a passenger vehicle collided with a tractor-trailer and got stuck. A woman and a teenage girl were killed and two other passengers were extricated and transported to trauma centers, according to the Casa Grande Fire Department.

Spokeswoman Barbara Rice said there was thick fog at the time of the accident.

Police also responded to an accident on westbound Interstate 10 and Alvernon in Tucson but said it could not be definitively linked to fog.

National Weather Service meteorologist John Gluck said the area sees fog like this only once every two years.

"I've been here 20 years and this is one of the better ones I've seen," Gluck said.

Tucson residents took to social media to post pictures of saguaros behind thick fog.

The fog was so dense it limited visibility to nearly zero in some areas. At the Tucson International Airport, visibility was down to one-quarter of a mile.

State Rep. T.J. Shope, of Coolidge, said he couldn't see more than two houses down when he left home Thursday morning. Shope said the fog added 15 minutes to his commute to Phoenix but cleared up once he got on Interstate 10.

The fog began clearing up in the early afternoon.