NEW ORLEANS — A high-ranking BP executive testified Tuesday that the London-based oil giant and its contractors share the responsibility for preventing blowouts like the one that killed 11 workers and spawned the nation’s worst offshore oil spill in 2010.

Lamar McKay, who was president of BP America at the time of the Deepwater Horizon disaster, is the first BP executive to testify at a federal trial designed to identify the causes of BP’s Macondo well blowout and assign percentages of blame to the companies involved.