A homeless woman sleeping in a cardboard box was struck and killed by heavy machinery operated by a road crew clearing a homeless camp, police said Wednesday.

Shannon Marie Bigley, 33, was killed Aug. 1 in a grassy field alongside a highway where a homeless camp was built, the Modesto Bee reported.

The incident occurred while a California Department of Transportation crew was attempting to clear the encampment about 90 miles east of San Francisco, Modesto police chief Galen Carroll said.

The investigation was turned over to the California Highway Patrol after police detectives determined the incident wasn't a homicide, the chief said.

Caltrans and the CHP provided no further details, citing the ongoing investigation.

In an email, Caltrans spokesman Chantel Vernon expressed sympathy for the family of Bigley.

The Stanislaus County coroner said Bigley was from Oregon, her last known address was in Stockton, and she recently began staying at the encampment in Modesto.

Four months ago, state officials rejected a union grievance filed by workers who object to clearing homeless encampments. The union argued that workers lack safety equipment and training.

The union filed a new grievance Wednesday. Union chief Steve Courch said it seeks a halt to the homeless camp cleanups by Caltrans maintenance workers.

"If you look at their job classification, nowhere is it mentioned that they clean up homeless encampments," Courch said.

The Bee reported that the homeless camp remained standing Tuesday.