TRAVERSE CITY, Mich. (AP) — Michigan officials are demanding an explanation after being notified by Enbridge Energy Partners that more steel supports are needed for twin oil pipelines running beneath the waterway that links Lakes Michigan and Huron.

The Straits of Mackinac pipes are part of Enbridge’s Line 5, which carries nearly 23 million gallons of oil and liquid natural gas daily to Canadian refineries.

The underwater pipelines rest directly on the lakebed in some places. In others, they’re supported by steel anchors. Enbridge says a recent inspection found that more anchors are needed to meet state requirements. The company is seeking permits to install them.

In a letter Wednesday, the Michigan attorney general’s office describes the shortage of anchors as a violation of Enbridge’s pipeline easement and requests information about why it happened.