A worker killed Thursday at the Sewickley sewage treatment plant died from injuries he suffered from falling about 30 feet into an underground shaft and not as a result of toxic fumes that flooded the hole where he was working.

In making that determination Friday, the Allegheny County medical examiner's office also ruled 31-year-old John Hogan III's death an accident.

But questions remained about what happened in the minutes before Mr. Hogan's death, and whether he and three coworkers who were sickened when they rushed into the 30-foot hole to help him complied with standards recommended for working in confined spaces.

Investigators from the federal Occupational Safety and Health Administration and officials with Allegheny County and the borough on Friday offered few glimpses into their probes of the accident, but said they were trying to learn whether safety protocols were followed. The federal agency is involved because at least two of the injured workers are employed by private contractors, an OSHA spokesman said.

Mr. Hogan, a borough employee for less than two months, had been working in the shaft on the plant's site just before 1 p.m. He was trying to climb out but fell back into the hole, borough officials have said, apparently overcome by toxic fumes, possibly methane gas.

Plant superintendent Dennis Mike; an inspector from the borough's engineering firm, Shawn Haynes; and a contract worker, Franklin Pounds Jr., climbed into the hole to help him and were overcome within seconds.

Firefighters with breathing apparatuses, ropes and harnesses pulled them to safety within 30 minutes. Mr. Pounds, 43, of New Castle, was the only man who remained hospitalized Friday. He was in serious condition at Allegheny General Hospital, recovering in intensive care from injuries his relatives said include broken ribs, a collapsed lung and fractures on his vertebrae.

The plant remains operational while employees from Gibsonia-based Optimus Risk Services study safety at the facility, said borough Manager Kevin Flannery.

"They've been all over the place," reviewing "everything from top to bottom," he said, but did not elaborate on their findings. Representatives from the company could not be reached for comment Friday afternoon.

"We're reviewing all of our procedures," Mr. Flannery said. He said he did not know whether the employees were equipped with breathing apparatuses or whether they needed them.

Such apparatuses are among several industry standards proposed by OSHA for work in confined spaces, where environmental conditions can change rapidly, especially air quality.

Sewer work usually requires an OSHA permit because of the many hazards. Among other things, OSHA regulations require a plan that equips rescuers with breathing apparatuses and ventilates the rescue area with fresh air.

The sewer site should be under constant monitoring for dangerous gases in a confined space at least 4 feet deep. Prospective rescuers in an emergency should be tethered to a lifeline before attempting to help fallen workers in a confined space, such as a sewer line, according to OSHA.

It wasn't clear whether Mr. Hogan and his co-workers followed these standards.

Allegheny County Emergency Services Chief Robert Full, through county spokeswoman Megan Dardanell, declined to comment, saying more information would be forthcoming next week.

The county health department inspects the plant and has jurisdiction in plant operation matters, but is not the entity responsible for investigating the accident. The county last inspected the Sewickley plant in July 2009 and found "no outstanding issues," health department spokesman Dave Zazac said. The plant has no history of problems, he added.

Construction will continue Monday on a $6 million expansion of the plant, situated near the banks of the Ohio River in the borough, Mr. Flannery said.

The state Department of Environmental Protection helped arrange funding for the expansion through the American Recovery and Reinvestment Act. But the DEP has no jurisdiction over operations of the plant, spokeswoman Helen Humphries said.

On site Friday were grief counselors, whose services will be available throughout the week. Mr. Flannery said employees were availing themselves of their counseling.

Working side by side each day, employees at the plant became close friends, workers said. Mr. Hogan was hired in Sewickley on June 1 after operating a sewage plant in Baden, Mr. Flannery said.

"He wanted to advance his career and learn more," he said. "He was very excited that he made the final cut."

Among other relatives, Mr. Hogan leaves behind an 8-week-old daughter. Funeral arrangements were pending.

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