“We must take into consideration how exposure like this might raise our risk profile, regardless of the remoteness of something actually happening,” Jeff Pounds, a spokesman for Williams, wrote in an e-mail.

The tower, the tallest building in Oklahoma, cried for attention when it was built 35 years ago in a decaying part of downtown. Originally, there were two smaller towers in the plans by Minoru Yamasaki, the architect who designed the World Trade Center. But shown a model, the head of the Williams Companies picked up one of the towers and stacked it on the other. The end result was a single 667-foot building — nearly half the size of the twin towers.

The building, part of a much larger complex, was completed in 1976 at a cost of $86 million, three years after the World Trade Center. The resemblance between the towers stretched from the arched plaza at the base to the boxy windows at the top and along the long vertical lines that defined the facade.

Dale A. Gyure, an associate professor of architecture at Lawrence Technological University in Michigan, who is writing a book about Yamasaki, said the tower was the most similar of several around the country that shared features with the World Trade Center, including buildings in Buffalo, Minneapolis, Seattle and Richmond, Va.

“He sort of developed a formula after the World Trade Center for his taller buildings,” Mr. Gyure said. “They all belonged to a family visually, but also structurally.”

Local news articles from the time make no mention of any comparisons with the gleaming new buildings to the east, but one, headlined “Williams Plans Against ‘Towering Inferno,’ ” declared that the building would be “one of the safest office towers in the world when completed.”

On the morning of Sept. 11, 2001, that sense of invulnerability was shattered. After the plane strikes, some employees at Williams reached out to employees at Cantor Fitzgerald, a business partner, who described the chaos around them as they waited to be rescued. When it became clear that help would not arrive, a few started giving messages to be conveyed to their children and spouses.