New London — The mysterious four-story structure that was built on a barge at State Pier left the city Wednesday.

A tugboat from Portland Tugboat LLC led the barge down the Thames River around noon. The Portland, Maine-based company, which is owned by McAllister Towing & Transportation, is escorting the barge to Portland.

An employee of the company confirmed the barge would arrive in Portland Thursday but said she did not know what the structure was.

Rumors swirled for weeks while construction was underway at the Admiral Harold E. Shear State Pier about what the structure could be.

Several people familiar with the project either said they did not know all of the details or weren't willing to divulge them.

Theories included a movie set, a floating prison, a Defense Department project, condominiums or office space.

The latest theory was that the structure is a new vessel for cargo service between Portland and New York that McAllister and the Maine Port Authority won federal funding to design. But Patrick Arnold, the port authority's director of operations and business development, said Wednesday that the vessel is only in the design phase, so it was not the structure built in New London.

"It's definitely not the tug-barge hybrid," he said.

The Connecticut Department of Transportation is responsible for the pier, and Logistec USA leases pier space from the DOT as one of two facility operators.

A spokesman for the terminal operator said Turner Construction leased space from Logistec around May to build the structure on the barge but never told Logistec what it was building.

Chuck Beck, the transportation maritime manager for the DOT, said last month the department was not involved in the fiduciary agreement between Logistec and Turner Construction and, as a result, "has little to no details on what was being constructed, why or for whom."

Turner Construction has not yet responded to requests for information about the construction. The Day has filed a Freedom of Information request for Coast Guard documents on the project.

j.mcdermott@theday.com