NEW ORLEANS — On the first day of testimony in the BP Gulf of Mexico oil spill trial, BP’s top executive for North American operations at the time of the disaster acknowledged on Tuesday that a well explosion had been identified as a risk before it happened.

“There was a risk identified for a blowout,” said Lamar McKay, the former president of BP America and current chief executive in charge of global upstream operations. “The blowout was an identified risk, and it was a big risk, yes.”

Robert Cunningham, a lawyer for private plaintiffs, tried to pin down Mr. McKay on BP’s responsibility for the 2010 disaster that killed 11 workers and dumped millions of barrels of oil into the gulf. Mr. Cunningham suggested that the British company’s cost-cutting and risk-taking culture were at the heart of the explosion and spill. He pressed Mr. McKay on the fact that a BP report on the accident held contractors responsible, but did not cite management failures.

Mr. McKay repeatedly responded that BP was responsible for designing the well, but that the rig, cement and other contractors shared responsibility for safety on the drilling operations.