WASHINGTON — People familiar with the investigation of the Gulf of Mexico oil spill say manslaughter and perjury are among the possible violations being explored by Justice Department investigators. Those familiar with the inquiry said the Justice Department is not ruling out the possibility of bringing manslaughter charges against companies or managers responsible for the explosion aboard rig Deepwater Horizon that killed 11 workers last April. The sources added that the department also is examining congressional testimony by company executives, including former BP CEO Tony Hayward, to determine whether their statements were untruthful. They cautioned that the investigation is far from complete and spoke on condition of anonymity about the ongoing investigation.