Federal authorities investigating BP PLC's oil spill in the Gulf of Mexico are zeroing in on bad decisions, missed warnings and worker disagreements in the hours before the April 20 inferno aboard the Deepwater Horizon that spawned one of the worst environmental disasters in U.S. history.

In particular, the panel is examining why rig workers missed telltale signs that the well was close to an uncontrolled blowout, according to an internal document assembled by the investigators and reviewed by The Wall Street Journal. The...