A worker assembles a box for delivery at the Amazon fulfillment center in Baltimore, Maryland, U.S., April 30, 2019.Clodagh Kilcoyne | ReutersFederal officials are sharpening their focus on Amazon's workplace safety hazards.Investigators from the Occupational Safety and Health Administration, which is part of the Labor Department, visited Amazon sites near Albany, New York, Denver and Boise, Idaho, on Monday, according to a document viewed by CNBC. The inspections are part of an ongoing probe launched last month by OSHA and the U.S. Attorney's Office for the Southern District of New York. OSHA officials in mid-July visited three other Amazon warehouses in New York, Chicago and Orlando in response to referrals from SDNY prosecutors. The U.S. Attorney's Office said last month that the civil division is investigating potential workplace safety hazards at Amazon warehouses nationwide, as well as possible fraudulent conduct designed to hide injuries from OSHA and other regulators. OSHA inspectors have so far zeroed in on Amazon's injury and illness recordkeeping, ergonomics program and Power Industrial Truck (known as PIT) operations, according to the latest document. PIT roles require employees to drive forklifts or other machinery to reach products on high shelves or move heavy items around the warehouse.Federal prosecutors asked current and former Amazon employees to report information about working conditions via an online form. One question on the survey reads, "Have you seen workers working in unsafe ways to try to meet their productivity/rate requirements?" Another says, "Do you believe that Amazon discourages workers from reporting injuries?"Representatives from OSHA and Amazon didn't immediately respond to requests for comment. Amazon spokesperson Kelly Nantel previously told CNBC in a statement that the company intends to cooperate with OSHA throughout its investigation. "We believe it will ultimately show that these concerns are unfounded," Nantel said.Amazon has repeatedly come under fire for its treatment of warehouse and delivery workers. Critics and lawmakers have pointed to the company's obsession with speed and strenuous quotas as threats to worker safety. Employees often claim they're at risk of discipline and being fired if they fail to meet productivity quotas. Amazon has denied that it uses such quotas in its warehouses, and disputed reports of unsafe working conditions. OSHA investigators have inspected Amazon warehouses on numerous occasions over working conditions, including a tragic warehouse collapse and coronavirus-related health and safety concerns. When an OSHA violation is identified, it typically results in fines that are paltry compared with the hundreds of billions of dollars Amazon hauls in annually.WATCH: A closer look at Amazon's warehouse injuries
Amazon warehouses face expanded probes into safety hazards as investigators visit three more sites.