Oil giant BP PLC told congressional investigators that a decision to continue work on an oil well in the Gulf of Mexico after a test warned that something was wrong may have been a "fundamental mistake," according to a memo released by two lawmakers Tuesday.

The document describes a wide array of mistakes in the fateful final hours aboard the Deepwater Horizon—but the main revelation is that BP now says there was a clear warning sign of a "very large abnormality" in the well,...