CAIRO — 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.

“If you guys do not get here, we are going to die!” a diplomatic security agent then shouted to them over the radio, the commandos say in the book, and they left the base in defiance of the chief’s continuing order to “stand down.”

The book, titled “13 Hours,” is the first public account of the night’s events by any of the American security personnel involved in the attack. The accusation that the base chief, referred to in the book only as “Bob,” held back the rescue opens a new front in a fierce political battle over who is at fault for the American deaths.