PETROLIA, Pa. -- Officials said the toxic cloud of sulfuric acid that formed after an acid spill in Petrolia has dissipated, and residents are allowed to return home. The all-clear came at about 3 a.m. Sunday.

A corrosive liquid overflowed from a tank at a chemical plant in western Pennsylvania on Saturday, evaporating into a toxic cloud that snaked along the ground and forced about 2,500 to flee. At least three residents were believed to be injured.

A material called oleum, similar to sulfuric acid, leaked from a tank at the Indspec Chemical Corp. plant in Petrolia, said plant manager Dave Dorko.

"Our safety in both our personnel and in our community come at the forefront of everything we do," said Dorko. "We didn't do a good job of that today, and for that I apologize. The community and the people we let down."

Authorities were concerned about the potential for respiratory damage and skin burns, said Freda Tarbell, spokeswoman for the state Department of Environmental Protection.

Authorities went door to door to warn the 2,500 people living within 2 miles to evacuate. Residents were taken to shelters in nearby towns, and officials said they did not know when they would be allowed to return.

"We are going to get them back in their homes as soon as it's safe," said County Commissioner Dale Pinkerton.

Red Cross officials said they expected to shelter only 50 to 100 people overnight.

No injuries were reported at the plant, where the spill was contained and crews were trying to clean it up. An official at Butler Memorial Hospital, where authorities said three residents were taken, declined to comment.

The dense cloud was reported moving slowly west Saturday night, close to the ground. No further evacuations were planned, but residents in the path were told to remain inside, to close doors, windows and vents, to shut down any system that draws air from the outside and to put out any fires in fireplaces.

Petrolia is about 40 miles northeast of Pittsburgh.