WASHINGTON — Several days before the explosion on the Deepwater Horizon oil rig, BP officials chose, partly for financial reasons, to use a type of casing for the well that the company knew was the riskier of two options, according to a BP document.

The concern with the method BP chose, the document said, was that if the cement around the casing pipe did not seal properly, gases could leak all the way to the wellhead, where only a single seal would serve as a barrier.

Using a different type of casing would have provided two barriers, according to the document, which was provided to The New York Times by a Congressional investigator.

Workers from the rig and company officials have said that hours before the explosion, gases were leaking through the cement, which had been set in place by the oil services contractor, Halliburton. Investigators have said these leaks were the likely cause of the explosion.