WASHINGTON — The Obama administration has so far used broad strokes to describe how it will repair an intelligence system that failed to stop an attempted Christmas Day terrorist attack. This week, with Republicans newly emboldened, Congress starts pressing for specifics. FBI Director Robert Mueller, Homeland Security Secretary Janet Napolitano and other senior security officials head to Capitol Hill for two days of hearings about how purported Nigerian al-Qaida operative Umar Farouk Abdulmutallab allegedly was allowed to board a plane for Detroit armed with a bomb. A White House report this month outlined a series of missteps, including name misspellings, ignored warnings and oversights.