HELENA, Mont. (AP) — A federal investigation into last year's Yellowstone River oil pipeline spill is expected to wrap up next month now that regulators have received a key document that reveals the cause of the break, a pipeline safety official said Friday.

Chris Hoidal, the Western region director for the Pipeline and Hazardous Materials Safety Administration, said his agency last week received a metallurgical report from Exxon Mobil Corp. that shows the definitive cause of the July 2011 failure of the company's pipeline.

Hoidal, who was in Helena to update state officials on the investigation and pipeline improvements since the spill, said the report will remain confidential while the accident investigation is still open. He declined to say what caused the break, though he ruled out internal pipeline corrosion.

"There were no surprises in the report," he said.

Officials have previously speculated that the 12-inch line, which had been buried a few feet below the riverbed, was exposed by high water last spring and damaged by passing debris, but an official cause has not been released.

The pipeline break dumped 1,500 barrels of crude into the pristine Montana waterway and was quickly swept downriver toward North Dakota after it took people responding nearly an hour to shut down the line. Crews worked for months to collect oil from the shoreline, backwaters and logjams, but only about 1 percent was recovered.

Exxon Mobil has since buried that pipeline and two others crossing major waterways dozens of feet below the riverbeds using directional drilling technology.

Other companies also have identified where their pipelines might lay too shallow at river crossings and are burying them deeper. The company with the most pipelines in Montana, Phillips 66 — formerly ConocoPhillips — is either burying or upgrading the safeguards for nine pipelines in Idaho and Montana and has nine more on tap for next year.

PHMSA must now decide whether to impose penalties on Exxon Mobil. That decision and the accident investigation should be completed by the end of September, though whatever penalties are handed down will be in coordination with an Environmental Protection Agency probe, Hoidal said.

But that information may not be released to the public before November, he said.

Montana officials already have reached a $1.6 million settlement with Exxon Mobil in a combination of cash and supplemental environmental projects that are still being negotiated.

A state pipeline safety council also is expected to wrap up its work next month when it finalizes its recommendations to Gov. Brian Schweitzer on how future spills can be prevented.

A draft of those recommendations — general words of support for better funding for federal regulators and improving pipeline technology — were widely panned during the public comment period by people who said they did not go far enough.

Many of the commenters said the state should take a bigger oversight role of the pipelines crisscrossing the state.

Montana Department of Environmental Quality director Richard Opper said Friday that the Oil Pipeline Safety Review Council will rewrite the recommendations based on the public comments.

But the changes are unlikely to recommend that the state share regulatory authority of interstate pipelines with PHMSA.

The state doesn't have the money, the expertise or the legislative authority to do so, Opper said.