Acting swiftly to take responsibility for a deadly refinery blast this year, BP PLC blamed company managers and employees for an explosion at its facility in Texas City, Texas, that killed 15 contract workers and injured more than 170 others.In a preliminary but detailed report on the March 23 blast, BP concluded that its employees committed "surprising and deeply disturbing" mistakes. The findings represent a significant setback for the British oil giant and its chief executive, John Browne. Lord Browne has worked for years to position BP as an exemplar of good corporate behavior by calling for action to combat global warming and other initiatives.