A full complement of employees returned yesterday to the Indspec Chemical Corp. plant in Petrolia, but normal operations have been suspended until it is determined what caused the leak of a dangerous chemical that forced evacuation Saturday of 2,500 residents in four northeastern Butler County communities.

Plant manager Dave Dorko said the facility was operating with its normal weekday daylight shift of 160 employees yesterday but there was no production of resorcinol, a chemical compound used in the tire and rubber industries. Indspec's Petrolia plant is the only facility in North America to produce resorcinol.

"The focus of our efforts today is the incident investigation," Mr. Dorko said. "Our people are trying to understand the cause of what occurred Saturday and, more importantly, to take measures to keep it from happening again."

He said production would not restart "until we're comfortable" that those goals had been accomplished.

Plant personnel and others from the corporation were trying to determine how and why a 38,000-gallon tank of liquid oleum, a toxic chemical used to make resorcinol, overflowed through a top vent about 4:30 p.m. Saturday. Alarms were sounded and the 30 employees working at the plant during that shift were evacuated with no injuries reported.

The spilled oleum, also known as fuming sulfuric acid, reacted with water vapor in the air, creating a dense cloud that hugged the ground and spread from the plant into the community for more than a mile. In high concentrations, oleum can be deadly; in lesser concentrations, it can irritate and damage the lungs.

Reacting to the emergency, Petrolia firefighters blocked off the highway into town and started going door to door to alert nearby residents to the evacuation that was soon extended to a 3-mile radius. About 200 evacuees went to one of three shelters set up at Karns City High School and the North Washington and Bruin fire departments. Some residents stayed inside their homes, and many others temporarily stayed with friends and family outside the evacuation zone.

By 3 a.m. Sunday, the cloud had dissipated and the evacuation notice was lifted for Parker, Fairview, Washington and Concord townships.

During the evacuation, three people, including two elderly women and a 44-year-old man in poor health, complained of respiratory problems and were taken to hospitals. It was not known if their symptoms were caused by the chemical cloud.

In addition to Indspec's internal probe, the state Department of Environmental Protection is investigating the incident. A spokeswoman was unavailable yesterday, which was a state holiday.