The so-called Google barge left Portland Harbor “for good” on Monday evening, according to the president of the tugboat company that escorted the barge to Boston Harbor.

Capt. Brian J. Fournier, president of Portland Tugboat LLC, said in a telephone interview late Tuesday night that the four-story, white-colored structure that has been sitting on top of the barge since it arrived in Portland will be scrapped at an undisclosed location.

The barge will remain in Boston for the time being before it is eventually put to commercial use somewhere in the Gulf of Mexico, Fournier said.

“It’s completely hollow inside. You can look up at the ceiling from the bottom floor. It’s empty,” Fournier said.

The only evidence that the structure was going to be used was a heating and air conditioning system that had been installed.

“I’m not sure what the excitement was all about,” Fournier said. “To us it was just another barge.”

The Google barge was moved in late July from the Portland side of the harbor – off outer Commercial Street – to Turner’s Island Cargo Terminal in South Portland.

The barge carried 63 shipping containers that had been connected to create a four-story building. The structure was assembled in New London, Connecticut, and towed to Portland, where Cianbro Corp. was scheduled to do interior work on it, including the installation of undisclosed technology equipment.

Cianbro has never disclosed the identity of the barge’s owner or its purpose. A similar mystery barge appeared on the waterfront in San Francisco last year.

After some digging by reporters on both coasts, Google admitted that it had commissioned the barges to serve as “an interactive space where people can learn about new technology.”

Roger Hale, owner of the South Portland terminal, later disclosed that the vessel had been purchased by an unnamed “international barging company.”

And on Monday, the barge and structure left Portland – never to return. The Casco Bay Bridge was raised at 5:41 p.m. to let the barge pass underneath.

“Rowan taking the barge formerly known as ‘Google’ out of Portland Harbor for good” was the caption underneath a photograph posted on Portland Tugboat LLC’s Facebook page.