U.S. Chemical Safety Board joins OSHA in federal investigation.

A fourth person has died from injuries sustained when a boiler described as weighing a ton exploded at the Loy-Lange Box Co. in St. Louis on April 3, sending it 500 feet and crashing through the roof of another business.

Clifford Lee, 53, was with two friends filling out new employee paperwork at Faultless Linen Co. Lee, an employee at the laundry business and noted for his previous work at a homeless shelter, died from his injuries April 6. Both his friends, husband and wife Christopher Watkins, 46, and Tonya Suarez-Gonzalez, 43, died at the scene.

The explosion injured four with one person still in a hospital in critical condition.

According to Fire Chief Dennis Jenkerson, the boiler was hot when firefighters arrived on the scene. A third building was damaged when an 8-foot piece of pipe linked to the blast smashed through its roof, Jenkerson said.

Here’s what we know about the accident:

Federal Investigation

A three-person team from the U.S. Chemical Safety Board has joined the investigation along with the Occupational Safety and Health Administration.

CSB Chairwoman Vanessa Sutherland said in a news release the board’s investigators will be examining "what, if any, safeguards were in place to protect the workers at the Loy-Lange Box Company, as well as for those in the adjoining building."

The CSB is an independent federal agency whose mission is to drive chemical safety change through independent investigations to protect people and the environment.

The agency was funded in 1998 and has investigated about 130 disasters across the country.

Safety Standards

The accident is calling into question how the city regulates the safety of such equipment.

St. Louis is exempt from a state law that requires regular inspections of high-pressure boilers by either a state inspector or an insurance company.

According to a story in the St. Louis Post-Dispatch, the city instead requires companies to have licensed stationary engineers on site while a high-pressure boiler is in operation. The city administers the licensing process, which includes classwork and a requirement to pass a test.

Also killed in the accident was Kenneth Trentham, 59, one of three stationary engineers employed by the box company to tend to the business’ two boilers.

A spokeswoman for the mayor’s office said the company was up-to-date with the city licenses, and that Trentham had been authorized for the work since 1996.