TRAVERSE CITY, Mich. -- Recent damage that prompted the temporary shutdown of an Enbridge oil pipeline in a channel linking two of the Great Lakes may have been done by vessels working for the company, according to an in-house report provided Thursday to The Associated Press.

Enbridge this week submitted to government agencies the results of its investigation into "disturbances" that were discovered during inspections of dual pipelines that run across Michigan's Straits of Mackinac. The two pipelines are part of Line 5, which carries oil and liquids used in propane between Superior, Wisconsin, and Sarnia, Ontario.

The Canadian company said it had narrowed down a list of vessels known to have been operating in the area that might have dragged a mooring cable across the pipelines, bending a support structure and scraping protective outer coating.

Four of the five likeliest suspects were Enbridge contractors doing maintenance or geotechnical work related to the company's plan to build a tunnel for a replacement pipeline beneath the straits, the report said.

The other was a dredging craft, it said, adding that efforts were continuing to determine what was responsible.

The damage was minor and didn't endanger either pipeline's structural integrity, the report said.

"Our investigators determined conclusively there was no internal or external metal loss, no dents and no safety issues with either the east or west segments and the pipelines are safe to operate," spokesman Ryan Duffy said.

Enbridge sent its findings to the U.S. Pipeline and Hazardous Materials Safety Administration, the Michigan attorney general's office and the state Department of Environment, Great Lakes and Energy. The state agencies said they were studying the document and had no immediate reaction. The federal agency did not immediately respond to a message seeking comment.

Michigan Attorney General Dana Nessel has called repeatedly for the shutdown of Line 5, saying the 4-mile-long (6.4-kilometre-long) underwater segment is vulnerable to a rupture that could do catastrophic harm to the straits between Lake Huron and Lake Michigan.

Environmental groups and Native American tribes also want the pipeline decommissioned.

A state judge granted a request from Nessel to close the line June 25 after Enbridge reported the damage. The judge later allowed the flow to resume through the western line, while keeping the eastern line closed pending further inspections.

The Enbridge report said the nature of the damage and drag marks in the lakebed pointed to a cable or anchor from a small or "moderately sized" vessel. An anchor from one of the large commercial ships that transit the straits would have left a deeper gash, it said.

Enbridge says it has stepped up security measures in the straits since a 6-ton (5.44-metric tonne) barge and tugboat anchor struck Line 5 in April 2018. They have focused primarily on avoiding contact with larger vessels but are being expanded to prevent encounters with smaller ones, Duffy said. A patrol boat now operates in the area at all times, he said.