The incident, which police are calling a "very grave accident", happened in Seattle's First Hill neighborhood, near 9th avenue and James Street.
Surveillance cameras near the intersection are helping detectives figure out what happened.
"(The surveillance video) was able to give us enough information to figure out the who, what, where, when and why of who was driving the truck and who owned the truck," said Det. Drew Fowler of Seattle Police.
The truck belongs to CleanScapes, a waste management company, and the driver is a man who was working his normal shift.
"No one is under arrest at this time. We are still gathering evidence and examining all aspects of the investigation," said Fowler.
The driver was heading southbound on 9th Avenue around 3:30 p.m. on Thursday then made a right turn on James Street.
"The truck wasn't swerving, it wasn't speeding, it wasn't driving erratically," said a witness who only wanted to be identified as Christine.
Christine was walking along James Street when she saw the gruesome scene.
"I looked up and I saw a big, lime green truck and there was a woman who was trapped behind the back wheel of the truck on the passenger side," said Christine. "She was dragged about halfway down the block, she was dislodged and then the truck just kept driving."
Police said it's possible the driver didn't even know he hit someone.
"As I was calling 911 I heard people saying 'Chase that truck' or 'Stop that truck' and one man ran after the truck and sort of tried to flag down the truck, but I think the truck was too far gone at that point," said Christine.
CleanScapes posted a statement on its website that read in part "We are cooperating fully with the authorities to assist them in gathering all of the information they need to make a determination of the cause.
CleanScapes has contracted with the City of Seattle since 2008 but no one would comment on the driver's history.