ST. LOUIS -- ExxonMobil officials and regulators on Thursday were investigating the cause of a small oil leak from the Pegasus pipeline in Southeast Missouri -- the second leak from the 850-mile pipeline in a month.

The latest leak was discovered Tuesday near Doniphan, Mo. About 42 gallons of crude oil -- or roughly one barrel -- spilled into retiree Robert Cooley's yard.

"I thought a kid had poured some oil out there, but I could see it was spreading," Cooley, 86, said. "I said, 'There's something wrong. Call the oil company.'"

ExxonMobil spokesman Russ Roberts on Thursday said the cleanup was nearly complete. ExxonMobil is working with the U.S. Department of Transportation and other regulators to determine the cause of the leak, "but it appears a third party might be involved," Roberts said. He declined to elaborate.

The possibility that an "outside force" caused the leak is one of several things being investigated, said Damon Hill of the Department of Transportation's Pipeline and Hazardous Materials Safety Administration.

The Pegasus pipeline, completed in 1948, is 20 inches in diameter and runs from Nederland, Texas, on the Gulf Coast to the Patoka Oil Terminal Hub in Patoka, Ill. It carries 95,000 barrels per day when operational and is buried, on average, 24 inches below ground.

The pipeline already was out of service after a much larger breach near Mayflower, Ark., on March 29, but that doesn't mean there wasn't oil inside.

The Arkansas breach resulted in a spill involving about 5,000 barrels of oil. The leak forced evacuation of more than 20 homes and created significant concerns in Mayflower, a town about 25 miles northwest of Little Rock, Ark. ExxonMobil has said the spill didn't affect Mayflower's drinking water supply. But the pipeline runs through part of the Lake Maumelle Watershed, an area that drains into the main drinking water supply for hundreds of thousands of people.

Hill said the cause of the Arkansas leak remains under investigation.

Pertinent address:

Doniphan, Mo.