President Obama said Tuesday that a "mix of human and systemic failures" allowed a Nigerian student allegedly carrying explosives to board an airplane en route to Detroit on Christmas Day, and he vowed to quickly fix flaws that could have doomed a flight carrying nearly 300 passengers and crew members. The president and his top advisers now believe there is "some linkage" with al-Qaeda, and the administration is "increasingly confident" that the terrorist group worked with suspect Umar Farouk Abdulmutallab to secure the deadly chemical mixture that he took aboard Northwest Airlines Flight 253, a senior administration official said Tuesday....