Investigators are trying to determine what killed an employee at the Sewickley sewage treatment plant Thursday and sickened three of his would-be rescuers who rushed into a deep underground shaft where he had fallen 30 feet.

Although the investigation is not complete, officials said a toxic and pungent mixture of gasses, including methane, is likely to blame for the industrial accident, which happened about 12:40 p.m., while workers were performing routine maintenance on the narrow shaft at the plant on Ferry Street.

It was unknown whether John Hogan III, 31, of Baden, a borough employee, died as a result of the noxious fumes or the fall. The Allegheny County medical examiner's office planned an autopsy today.

It was the fourth industrial accident in the region in the last two weeks, which together have resulted in five deaths.

Injured were the plant's superintendent, Dennis Mike; an inspector from the borough's engineering firm, Shawn Haynes; and a contract worker, Franklin Pounds Jr. They hurried into the hole to help Mr. Hogan, who officials said was working inside but fell while trying to climb out.

"They were all overcome," said Mr. Pounds' father, Franklin Pounds Sr., who waited in the emergency room of Allegheny General Hospital Thursday night.

He said his son, 43, of New Castle, suffered broken ribs, a collapsed lung, fractures on his vertebra and other injuries and was recuperating in the hospital's intensive care unit.

A plant supervisor told Mr. Pounds his son fell into the shaft first, and was injured when his coworkers tumbled in after him. "He's in a lot of pain. It's going to be a while before he'll be out running again."

Mr. Mike and Mr. Haynes, 33, of Bentleyville, were treated and released from AGH, where they were taken for its hyperbaric chamber, which provides intensive oxygen treatment, though none of them needed it.

"They were definitely very lucky," Mr. Haynes' wife, Marla, said, while expressing her condolences. Working together daily, the men at the sewage plant had become close friends. "It's a big loss."

Firefighters, paramedics and a hazmat team arrived on the scene just before 1 p.m., when they received reports that four people were trapped inside the hole, 30 feet below the ground. They immediately began pumping the shaft with fresh air as firefighters moved inside.

They wore breathing apparatuses and brought extras for those who were trapped. Through a system of ropes and harnesses, the firefighters pulled the men to safety by 1:18 p.m., about 30 minutes later, Sewickley Fire Chief Jeff Neff said.

The shaft is about 10 feet wide "a very tight situation to work in," he said. One firefighter was treated for minor injuries at the scene.

Initial rescuers reported Mr. Hogan unconscious while inside the shaft, said Valley Ambulance Chief J.R. Henry.

"He never regained consciousness," he said.

Mr. Hogan was the father of an 8-week-old daughter, said borough Manager Kevin Flannery, who personally informed his mother and wife of his death.

"The employees in Sewickley are a close group," Mr. Flannery said. "We're all taking it pretty hard."

Although work halted at the plant on Thursday while investigators examined the scene, it is still in operation.

Guillermo Cole, a spokesman for the Allegheny County Health Department, which inspects all sewage treatment plants in the county, said the department has dispatched an employee to the plant to help evaluate what happened and offer assistance.

"We don't think this will have any effect on their operation," he said of the accident.

Personnel from the federal Occupational Safety and Health Administration were also called to the scene.

Allegheny County Emergency Services Chief Robert Full said investigators will try to recreate conditions inside the shaft to determine what kinds of fumes might have mixed inside and what happened just before the accident. Methane and hydrogen sulfide were likely among the "pungent and toxic gasses," he said, adding that it was unclear where or how they originated. The plant was evacuated, but workers remained nearby to answer investigators' questions.

"Confined space accidents injure and kill people almost daily in this country," Chief Full said.

Mr. Hogan was performing a task he did daily. His coworkers thought he had simply fallen and had no way of knowing about the fumes, Ms. Haynes said.

This past winter, the Sewickley Water Authority, owned by the borough, began working on two projects at its treatment plant that are expected to cost $6 million and take two years to complete.

One of the projects is designed to improve its headworks, which grinds up the solids that come into the plant. The other is construction of a dewatering facility, to remove water from wastewater sludge, in an effort to reduce odors.

Construction will continue on the upgrades, Mr. Flannery said.

The water authority serves customers in Sewickley, Aleppo, Bell Acres, Edgeworth, Glen Osborne, Haysville and Sewickley Heights.

Thursday's was at least the fourth major industrial accident in the region in recent weeks. On July 23, two workers were killed in an explosion at an oil and gas well in Indiana Township. A day earlier, two men were killed in an accident at the Horsehead Corp. zinc plant in Beaver County. And on July 14, 20 workers were hurt when coke-oven gas ignited at U.S. Steel Corp.'s mammoth coke manufacturing plant in Clairton.

"There's no commonality," Chief Full said. "It's really been a tragic two weeks."

Looking for more from the Post-Gazette? Join PG+, our members-only web site. You'll get exclusive sports content, opinion, financial information, discounts from retailers and restaurants, and more. Our introduction to PG+ gives you all the details.