The deadly accident happened around 7 a.m. Monday.
Police have identified the truck driver as a 52-year-old Gilbert Acuna from Rialto. Acuna was rushed to Community Regional Medical Center after the accident. Acuna died from internal injuries a few hours later.
Sgt. Anthony DeWall of the Fresno Police Department's Traffic Bureau, says investigators tried to recreate what happened leading up to the accident.
"It's one of two possibilities," DeWall said. "Either the brakes failed or he forgot to set the brakes, which is unfortunate."
Investigators had a big-rig mechanic take a look at the truck's break system.
"They have a compressor, that's how the lines are charged," DeWall explained. "That's what engages the solenoid on the brakes to keep them locked."
DeWall said when investigators set the brakes on the truck, it worked as it should have. There was no sign of any faulty parts.
"There wasn't anything obvious," DeWall said. "There wasn't air leaking or anything like that, but it is mechanical so things can happen to it."
DeWall says the truck and driver are with a small delivery company out of southern California. He and his partner had left Saturday evening for a route in Oregon and the Bay Area.
They were headed home when they got rerouted to make deliveries in the Valley.
At the time of the accident ? his partner had gone to the front of REI to get employees to help them unload. He told police He had no idea why Acuna went to the back of the truck. "(Acuna's Partner) said they were getting ready to back the semi-truck into the loading dock," DeWall said. "He thought everything was fine when he left the driver with the truck and walked to the front of the store."
Investigators will be checking its maintenance and service records. The truck has been released back to the delivery company in southern California.