LAWRENCE — The Duck Bridge over the Merrimack River, one of the city's busiest vehicle crossings, will remain closed to traffic at least until Thursday and possibly longer as state engineers devise a plan to fix the historic structure after seven of the horizontal trusses above the roadway were struck Monday by an oversized vehicle, bending them and in some cases ripping them from supports.

Around 11 a.m. Tuesday, according to Mayor Dan Rivera,  a citizen reported to MassDOT the damage of an overhead beam on the Duck Bridge.

Police found that several twisted beams on the bridge were hanging down over the roadway. It was clear they had been hit by a high-clearance vehicle heading south on Union Street, from downtown Lawrence toward South Lawrence.

Police, Public Works and Traffic Control workers, under guidance from MassDOT, closed the bridge at both ends as a safety precaution.

A MassDOT inspector was dispatched to inspect the bridge while a construction crew from SPS New England of Salisbury showed up to begin repairs. However, it appears repairs may have to be delayed.

Rivera said in a statement Tuesday night that engineers "continue to assess the damage and devise a repair, taking into account that these truss segments are not standard and would have to be custom built (due to the age of the bridge)."

"All signs point to the bridge remaining closed overnight and through (Wednesday)," Rivera said. It looks like it will also be closed Wednesday night as well, Rivera added.

In the meantime, MassDOT has provided signs indicating detours. The state agency will also provide an update on the matter Wednesday morning.

Police Chief Roy Vasque said security cameras caught the offending vehicle — a truck hauling an excavator on a trailer — but he wouldn't release the name of the company, which is doing business in the city, deferring instead to Mayor Rivera. As of Tuesday night, Rivera had not released the name of the contractor.

Vasque said the truck hit the bridge beams around 1 p.m. on Monday, according to the video recording, which came from a camera on the north side of the bridge. In addition, he said, several witnesses came forward identifying the truck involved.

According to police and MassDOT officials on the scene Tuesday, the vehicle struck seven overhead beams, just denting some while pulling others away from the bridge structure.

At their lowest point, the beams are about 14 feet above the surface of the bridge. The lower part of the bridge did not appear to be damaged.

Officers detoured traffic onto Canal Street on the downtown side of the bridge and onto Merrimack Street on the south side of the bridge, presumably the same detours that will be used by the state.

The bridge remains open to pedestrians because concerns are only for its ability to support the weight of vehicles.

The bridge spans the river, connecting South Union Street in South Lawrence and Union Street in the northern part of Lawrence.

It was originally built in 1888 and most recently rebuilt in 2013. Also known as the Union Street bridge, the name Duck Bridge came from a nearby mill that made duck canvas, a heavy-duty textile used for everything from sneakers and painting canvases to tents and sandbags.