CINCINNATI (AP) _ A quick-thinking chemical plant foreman ''very likely saved a lot of lives'' when he sounded an alarm just seconds before explosions ripped through the building, killing one worker and injuring 68, authorities said.

''One of the maintenance foremen noticed a vapor cloud, immediately ran over and pulled the alarm,'' Ron Leisner, a quality assurance manager for BASF Corp., said at a news conference. ''He was very calm under pressure and very likely saved a lot of lives by doing so.''

Leisner said the foreman, Joe Brock, was outside the resin building when he saw the vapor, what Leisner described as being similar to the shimmering effect above a blacktop road on a hot day.

''Our fire alarm went off, I got about five steps ... The explosion hit and it knocked everybody off their feet,'' said Thomas Borst, a chemist who was working in an adjacent building at the plant when the blast hit.

Larry Jameson, president of BASF's Coatings and Colorants Division, said the company had not determined the cause of the explosion, but suspected that something went wrong with equipment that processes chemical polymers into paint.

He said the process is used at other BASF plants, and there were no plans to suspend those operations. ''We really have no reason to suspect that this is an unsafe practice,'' Jameson said.

Firefighters continued to douse the building with water today to prevent chemicals from reigniting, Fire Chief Bill Miller said. It took about 2 1/2 hours to bring the fire under control.

About 2,000 residents were evacuated from their homes Thursday night while inspectors checked for structural damage.

By this morning, Norwood Fire Chief David Cartuyvelles said, everyone was allowed to return to their homes. Only some businesses near the plant remained closed today, he said.

Most of the evacuees were in Norwood, a suburb surrounded by Cincinnati. The plant is on the line between the two municipalities.

''I thought an airplane hit the building,'' said Charles Moore, a resident who said he was in his 80s. ''The concussion knocked me out of my chair. It sounded like a bomb.''

The cause of the explosions in the building on the 9-acre site remained under investigation.

''We don't have any idea what started the blaze,'' BASF spokesman Helmut von Moltke said.

The first explosion sent a cloud of thick, black smoke pouring from a four- story brick building containing resins and solvents.

About 150 people were in the plant, which makes coatings for the insides of soft drink cans and for paper cups, said Kenneth Diedenhofer, a plant production manager.

Diedenhofer said the building was ''engulfed in flames. I saw one guy hanging from the steps outside the building. The steps were blown out from the wall. His leg was tied into the steps. I was able to get him out.''

A second blast about a half-hour later blew out storefront windows as far as a quarter-mile away in the neighborhood of small, two-story homes. Smaller explosions sent balls of fire rolling into the sky.

Malcolm Adcock, assistant city health commissioner, said preliminary findings by city investigators suggested that ''volatile and explosive'' industrial solvents leaked from a mixing vat and were ignited by a spark.

Police Chief Lawrence Whalen identified the dead man as Lawrence D. Krechting, 55, of Norwood, a BASF maintenance worker. He apparently was killed by flying debris in the building where the explosions occurred, Whalen said.

Hospitals in the city reported treating 68 people for burns, broken bones and cuts caused by flying glass.

Three people remained in critical condition today.

The company has accounted for all of the workers who were in the plant, Miller said. Tests by the city and by the Ohio Environmental Protection Agency showed no toxic fumes in the smoke from the fire, Miller said.