Wednesday marks the first anniversary of the explosion on the Deepwater Horizon oil rig in the Gulf of Mexico. The accident killed 11 workers, shut key fishing areas for months and resulted in one of the worst oil disasters in the history of the industry.
Parts of the southern coast of the United States are still reeling from the accident.
The well was sealed off in July and "killed" in September but not before several failed interventions and millions of barrels of oil leaked into the water.
Exxon Mobil Chief Executive Officer Rex Tillerson told the Financial Times that BP officials knew a containment dome, one of the first techniques tried to stop the leak, "wasn't going to work."
Tillerson said it was known at the time that ice buildup at the sea floor meant the first interventions would fail and BP lost time while the integrity of the Macondo well "was deteriorating daily."
"So I think there was some time lost pursuing some alternatives that most of us felt that in the end were not going to be successful -- and with the loss of time the challenges just got greater," he added.
Washington lifted a moratorium on deep-water drilling in October and has since doled out 10 new exploration permits in the Gulf of Mexico. Tillerson said that was far from a business-as-usual scenario, however.
BP in a response said it made "all efforts" to contain the leak.