In making the announcement, the federal workers' safety agency said it also had opened an investigation into a second death, at an Amazon warehouse in Pennsylvania just two weeks ago.
The Occupational Safety and Health Administration (OSHA) cited five companies for "serious violations" of worker safety rules after an investigation into the death of Ronald Smith in New Jersey last year.
The violations carry potential fines of just $6,000 for each company.
Amazon is not named in the investigation. The company operates about 90 warehouses around the U.S., staffed mostly by laborers hired and managed by subcontractors.
- Ronald Smith, who was 57, died of injuries received when he was crushed in a conveyor system while sorting packages at a warehouse in Avenel, New Jersey last December. He had four children and seven grandchildren.
- Jodi Rhoads, 52, was crushed on June 1 when the pallet jack she was operating at a warehouse in Carlisle, Pennsylvania, crashed into shelving, pinning her between the two. A widow and a breast cancer survivor, she left behind two children and four grandchildren.
The statement released Friday said four staffing agencies and a logistics company were cited for serious violations, defined as those with "substantial probability that death or serious physical harm could result from a hazard about which the employer knew or should have known."
The rules allegedly violated are related to proper assessment of risks to employees.
The agency said a new investigation was being undertaken into the death of Rhoads in Pennsylvania.
The companies are subcontractors for Amazon. The web retail giant operates about 90 fulfilment centers around the U .S. alone. The network of regional warehouses, famed for its efficiency, is designed to speed delivery of orders to customers. Worldwide, about 70,000 temporary workers were hired to handle the holiday rush last December.
Amazon released a statement saying "Any accident that occurs in a facility is one too many and we take these matters seriously."