justice dept. sent 7-page letter to house speaker john boehner explaining why the former irs official was allowed to plead the fifth amendment .
federal prosecutor in charge of the case sent his decision to capitol hill on the last day before his own resignation took effect .
lerner offered a self-serving opening statement during a 2013 hearing , but refused to take questions even though she was under subpoena .
hearing focused on the irs 's habit of targeting conservative groups with special scrutiny based on words like ` patriots ' or ` tea party ' in their names .
the doj says she ca n't be prosecuted for defying the subpoena ` because she made only general claims of innocence ' and offered few details .