A small number of dolphins that washed up in the Gulf of Mexico have oil on them from a blown-out BP PLC well, but it isn't clear whether the oil was a factor in their deaths, federal scientists said Thursday.

National Oceanic and Atmospheric Administration officials said 293 dolphins—an abnormally high number—have washed ashore along the Gulf coast since late April 2010, when an explosion aboard the Deepwater Horizon, a rig drilling a BP well, sent millions of barrels of oil into the Gulf. Most of the dolphins were dead,...