NEW YORK – As the United States struggles to understand last September’s attack on its diplomatic mission in Benghazi, which took the lives of four Americans, including US Ambassador J. Christopher Stevens, a formal investigation has not even been opened in Libya – and likely never will be. The country’s leaders face myriad challenges – from a vocal federalist movement in the East, aimed at usurping the central government’s prerogatives, to a wave of assassinations targeting security officials – which leaves them few resources to allocate to a case that poses no immediate threat to their domestic standing.