At least two people suffered major injuries in a collision just south of the Redwood landfill on Highway 101 that fouled Wednesday evening commute traffic.

The crash occurred around 2:30 Wednesday afternoon when a man driving a pickup truck in the southbound lane was alerted by passing driver that there was a loose item in the back of the truck and he pulled over to the highway's right shoulder to secure it, a Marin CHP official said in a preliminary report.

The vehicle's passenger, 30-year-old Aaron Mudrich of Point Reyes, got out of the truck to secure the item, while the driver, 38-year-old Jared Sperry of Point Reyes, apparently stayed in the truck.

While the truck was pulled to the side of the highway, a box truck approaching in the lane closest to the shoulder and attempted to merge into another lane to give the truck more room, CHP officials said.

The driver, 30-year-old Javier Rodriguez of Alameda, reported hearing a loud bang from under the box truck before the truck suddenly turned hard to the right.

The box truck collided with the pickup, causing major injuries to both Mudrich and Sperry.

Many witnesses stopped to help out and confirmed that they had also heard a loud bang before the accident, CHP officials said.

Emergency personnel from the CHP, Novato Fire Department, and the Marin County Sherriff responded to the scene and took Mudrich and Sperry to Marin General Hospital.

A southbound lane was closed untul about 3:45 as emergency workers investigated the crash, and traffic was crawling through Petaluma as late as 5 p.m.

Officials do not believe that alcohol played a role in the crash.