Several black zinc panels on U.S. Bank Stadium came loose during Tuesday night's storm, and stadium officials were at a loss to explain why Wednesday.

Mortenson Construction vice president John Wood said the "extreme weather" caused the panels to partially disengage along their top edges, although none fell off. "The panels will not need to be replaced, just reinstalled, unless the panels themselves were damaged," Wood said.

Minneapolis-based Mortenson, the general contractor on the building, is working with the architects, Dallas-based HKS; the sheet metal subcontractor, Maplewood-based MG McGrath Inc., and other experts to understand why the panels are coming loose and how to fix them, Wood said.

Unsecured rectangular panels are more than a tangential concern because U.S. Bank Stadium's exterior walls are lined with thousands of them.

Minnesota Sports Facilities Authority (MSFA) Chair Michele Kelm-Helgen said it wasn't the first time the panels have come loose.

In a previous incident several weeks ago, the panels were believed to be loose because of other work to fix a moisture problem, but now the two appear unrelated, she said.

Since February, Mortenson has worked to address dampness on a parapet wall and some pooling of water in a gutter. The water had yet to seep into the building's interior walls, but the company was installing a new moisture barrier at an estimated cost of $4 million.

Replacing the old barrier required removing some of the black panels encircling the building's exterior.

Just as Mortenson paid to fix the moisture problems, Kelm-Helgen said the company will repair problems with the zinc panels at no cost to taxpayers.

Kelm-Helgen estimated that about 10 panels are loose. Although Tuesday's storm appeared to be a culprit, she said she couldn't be certain that was the cause.

She made the announcement about the panels to reporters gathered at the Vikings' offices across the street from the stadium for a news briefing on the traffic management plan for the new building. At least one loose panel on the eastern side was visible from the conference room.

Several of the loose panels are on the 270-foot west prow of the structure.

Vikings vice president Lester Bagley was sanguine about the situation, saying simply that the team is working with the MSFA to "get to the bottom of it" and find a "permanent fix."

The stadium's ribbon-cutting is just over two weeks away, on July 22. A free open house will follow that weekend. Kelm-Helgen said 80,000 tickets have been claimed for that event.

The Vikings will play their first regular-season game in the building on Sept. 18 vs. the Green Bay Packers.

Rochelle Olson • 612-673-1747 Twitter: @rochelleolson