Five commandos guarding the C.I.A. base in Benghazi, Libya, in September 2012 say that the base chief stopped them from interceding in time to save the lives of Ambassador J. Christopher Stevens and an American technician during the attack on the diplomatic mission there.

In a new book scheduled for release next week and obtained by The New York Times, the commandos say they protested repeatedly as the base chief ordered them to wait in their vehicles, fully armed, for 20 minutes while the attack on the diplomatic mission was unfolding less than a mile away.