A ProPublica story:

The head of the U.S. Senate's workplace safety subcommittee has asked the Occupational Safety and Health Administration to explain its handling of the death a temp worker who suffocated under a pile of sugar at a Pennsylvania plant. The details of OSHA's investigation were reported Sunday by ProPublica and Univision.

Sen. Robert Casey, D-Pa., wrote in a letter to the agency's director Thursday that he was "particularly troubled" by the death of Janio Salinas, who slid into a mound of sugar and was asphyxiated while breaking up clumps in a hopper in Fairless Hills, Pa. The plant had removed a safety screen just 13 days before the Feb. 25, 2013 accident because the plant manager believed it was slowing down production.

But OSHA inspectors did not find the company "willfully in violation," and subject to stiffer fines, despite the removal of the screen and previous fines for failing to train temp workers. The company was fined $25,855, but the penalty was later reduced to $18,098.

"While I appreciate that OSHA has limited jurisdiction in prosecuting workplace accidents, the growing number of accidents and fatalities involving temporary workers is clearly unacceptable," Casey wrote to OSHA director David Michaels.

Casey said he was also concerned about "possible regulatory or legislative impediments to OSHA's ability to ensure safe and healthful workplaces for temporary workers."

This is the hopper at the CSC Sugar plant in Fairless Hills, Pennsylvania, where Janio Salinas was buried alive in sugar while trying to break up clumps. (Courtesy of The Occupational Safety and Health Administration)

Stories that ran Sunday on ProPublica's site and on Univision's Aquí y Ahora news magazine show detailed how workers in the temporary staffing industry — one of the fastest growing sectors of the economy — face a significantly higher risk of getting injured on the job than permanent employees.

Last year, OSHA announced an initiative to raise awareness about the dangers facing temp workers and educate employers about the responsibilities they have when hiring temp workers. Since then, OSHA has stepped up its enforcement of rules affecting temp workers. But OSHA does not have the ability to shut down businesses and has limited criminal enforcement provisions.

A bill known as the Protecting America's Workers Act, which would raise fines and criminal penalties for OSHA violations, has been proposed in every Congress since 2004 but has never made it out of committee.

Casey asked OSHA for an update on what steps it has taken to protect temp workers, data on workplace incidents involving such workers, and what additional actions the Obama administration and Congress could take to improve safety.

OSHA spokesman Jesse Lawder said, "We share Senator Casey's commitment to the health and safety of temporary workers and look forward to working with the senator to find ways to improve conditions for this growing population of workers."