SACRAMENTO, Calif. (AP) _ At least 20 vehicles including a dozen big rigs were involved in a pileup on foggy Interstate 5 north of Sacramento today.

The accident started about 8:20 a.m. in northbound lanes about 10 miles north of downtown Sacramento. All lanes in both directions were shut down and traffic was diverted to other freeways.

Dense fog has reduced visibility to 30 feet or less, said Jim Harris, a spokesman for the California Highway Patrol.

There were injuries, but Harris could not say how many. He said authorities were trying to extricate victims from some of the vehicles.

A man who said he witnessed the aftermath saw two tanker big-rigs on fire, crumpled cars and trucks including recreational vehicles.

``It's the scariest thing you've ever seen,'' said the witness, who was not identified.

One CHP fire engine responding to the wreck rear ended another vehicle even though he was traveling slowly, the highway patrol said.