DOVER, Del. — A contractor dumped a mountain of dirt about two stories high and 100 yards long next to an interstate bridge over several years, so much that it may have moved the ground and caused the bridge to tilt, state officials said Wednesday.

The Interstate 495 bridge, a major East Coast thoroughfare traveled by 90,000 vehicles daily, was closed Monday when engineers determined that bridge support columns were leaning.

The contractor said he was working with state officials to remove the dirt from the site, which he was allowed to use under an arrangement with a company that leases land. However, state officials said some of the dirt appeared to be on state land and a fence cordoning off the government's property was gone.

Officials aren't sure when the bridge will reopen to traffic, which has been detoured to Interstate 95. No major problems have been reported.

"I really feel bad about what happened," said contractor James Thomas Jr., 60. "I have absolutely no idea what happened, I really don't. ... I'm not a structural engineer. I'm not a bridge engineer."

Engineers suspect the weight of the dirt caused the ground underneath the bridge to shift. Four pairs of the bridge's support columns were tilting toward the pile of dirt.

Officials have said a system to shore up and brace the bridge will have to be designed, which will take weeks. State officials do not have an estimated price tag but have indicated that they might seek federal funds to help pay for the repairs.

Delaware Transportation Secretary Shailen Bhatt said officials did not know about the dirt mound until Monday, when engineers visited the bridge in response to a report received late last week. That report came from an engineer with a private company who was in the area on an unrelated project and saw cracking in the soil around the dirt pile. The engineer then spotted the leaning columns and contacted the transportation agency.

Thomas said he has worked in the area around the Port of Wilmington, just down the road from the bridge, for 41 years.

"I used to unload and load ships. I had a paving business and some other businesses. ... I've worn a lot of different hats down there," he said.

Thomas said no one had ever expressed concern about him storing dirt next to the bridge. He said he sells it as fill dirt.

Bhatt said: "I don't know where he gets his dirt or what he uses it for.... Our primary focus right now is just getting it out."

The DuPont Co. owns the land where the dirt is located and leases it out to a materials handing company called Port Contractors Inc.

Thomas's company, Keogh Contracting, has an arrangement with Port Contractors, which was founded by his father, to store dirt on the property.

Michael Evanko, president of Port Contractors, said his company is allowing Thomas to temporarily store dirt being removed from the bridge site on another parcel just down the road.

Built in 1974, the bridge is scheduled for inspection every two years and was last examined in October 2012.

Officials said they were examining aerial photographs in an effort to ascertain how long the dirt mound has been there.

"In 2012, there was some stuff out there but not very much; in 2013, a little more," Bhatt said. "Right now obviously there's a lot more dirt under there."

Crews plan to erect a new fence at the site after enough dirt is removed. Bhatt didn't know what happened to the original fence.