Mr. Daniel said the company used internal inspection tools to determine the levels of corrosion or cracking on all pipelines. There had not been any repairs or replacements made to the part of the pipe where the spill took place, he said.

State officials here have expressed grave concern over the environmental impact if the spill reaches Lake Michigan, more than 60 miles away. Officials for the Environmental Protection Agency, which is leading the response efforts, said Thursday that they were confident that they could prevent that from happening.

Ralph Dollhopf, a federal coordinator for the agency, said efforts by response workers had so far stopped the oil from entering Morrow Lake, about 30 miles downstream from the site of the spill.

“We do not anticipate that Lake Michigan is at risk,” he said at a news conference.

Still, officials said it could take months to clean up the spill, which was believed to be among the largest ever in the Midwest. Hundreds of response workers continued on Thursday to lay boom, skim the water and conduct flyovers to assess the damage.