According to the Washington Post, all that is known at this time is that McNamara died at his home in Washington early this morning. He was 93.
A brief biography of McNamara can be found here.
It should be noted that McNamara, a man who relied on data and statistics much to his credit and discredit, was one of the higher ups who felt troop escalation was not the way to administer warfare and wondered if the war could be won at all. In other words, he was increasingly not down with tossing out blacks and poor whites to be cannon fodder in vain and resigned because of that tactical approach.