OAKLAND (AP) — Federal and state wildlife officials are investigating a tree-trimming operation in Oakland that witnesses say sent baby birds into a wood chipper, a newspaper reported.

Horrified onlookers called police on Saturday, as crews ground up black-crowned night heron bird chicks that were nesting in trees being trimmed in the downtown area, the San Francisco Chronicle reported on Tuesday.

“It was awful,” said Lisa Owens Viani, director of Raptors Are The Solution, who was among the first on the scene. “It’s especially appalling because these birds are so vulnerable and such a valuable part of the ecosystem.”

Police stopped the work, and Viani and others were able to rescue five chicks, according to the Chronicle.

The tree branches were being taken down by a company called Campos Tree Service at the request of the U.S. Postal Service. Postal officials were concerned about birds defecating on mail trucks, the Chronicle reported.

Joe Campos, a supervisor with Campos Tree Service, said the crew was new and didn’t know the baby birds were in there.

“It’s a big deal, though. We don’t want to destroy anything,” he said.

Postal service spokesman Augustine Ruiz said the incident was a terrible accident, and he understood why people were upset.

Black-crowned night herons are protected by state and federal laws and are left alone by city work crews in Oakland, the Chronicle reported.

The nest destruction may have violated the U.S. Migratory Bird Treaty Act. State and federal officials are investigating, said Andrew Hughan, a spokesman for the California Department of Fish and Wildlife.

“If we decide to pursue this matter, it would most likely be criminal and go through a district attorney,” he said.