CHICAGO – The leak from an oil pipeline near Chicago appeared to be slowing Saturday, and officials were hopeful they would be able to excavate the site soon to determine the cause and exact location of the break, a U.S. Environmental Protection Agency official said.

But how much oil has spilled and when the leak began remained unclear. The leak was discovered Thursday, and between 200 and 600 barrels of oil were being recovered each hour Friday, said EPA on-site coordinator Sam Borries. A barrel has 42 gallons.

Oil was no longer rising to the surface Saturday, and crews cut open pavement and dug about 5 feet to reach the 34-inch diameter pipe. Pipeline owner Enbridge Energy Partners — which also owns a pipeline that leaked hundreds of thousands of gallons of oil into a southern Michigan waterway in July — will drill through the top of the pipe to extract remaining oil, Borries said.

A company spokeswoman said the Illinois leak doesn’t appear related to the Michigan spill, but a federal official said it may be too early to tell. The EPA has ordered Enbridge Energy Partners, an affiliate of Canadian company Enbridge Inc., to stop all flow of oil by noon Monday.

The leak was discovered in the Chicago suburb of Romeoville by local water department workers responding to a complaint from a business owner about a drinking water line, Borries said. Oil was coming from a pipe that runs under a street through an industrial park and moves oil to a refinery about 1 to 2 miles east of the spill site.

Borries said there was no way to know how long the pipe had been leaking, but oil was rising to the surface and had run into a ditch and retention pond, and reached the town’s wastewater treatment plant. He said the oil was stopped at the plant, which was closed down for about 24 hours.

Saturday, oil had stopped surfacing, but officials did not yet know if oil was still left in the pipeline or even the exact location of the break, he said. Crews were excavating on both sides of the suspected leak location.

“We need to visually see the pipeline,” Borries said. He was unsure how much oil had been recovered, and said the EPA has requested that information from the company.

Enbridge still doesn’t know what caused the leak or how much oil has spilled, company spokeswoman Terri Larson said Saturday. Enbridge hoped to get a better look by late Saturday at the damaged steel pipe, which is surrounded by utility lines, so “there’s a lot of congestion around and we are being careful not to do anything to make things worse,” Larson said.

At this point, Larson said, Enbridge doesn’t believe the leak in Romeoville had anything do with the rupture in another pipeline in Marshall, Mich., that spilled at least 800,000 gallons of oil into a creek.

But Matt Nicholson, an investigator with the National Transportation Safety Board, said it’s too early to say whether the leaks are related. The government, not the company, will have the final say, he said.

“We’re going to make that determination whether it’s related to Marshall or not,” Nicholson said. “If the mechanism of failure is similar to what we find in Marshall, then it’s going to be something that we’re interested in seeing. Even though they’re different lines, it could be the same root cause.”

The NTSB, which investigates pipeline leaks, probably wouldn’t be involved with the Romeoville leak if it didn’t have so many similarities to Marshall, including the same company and pipes around the same age and size, Nicholson said.

The investigation into both leaks could take several months, he said.