The U.S. Department of Transportation (DOT) published notice RSPA-99-5143 (notice 99-11) stating that its files contain reports of 12 explosive failures of aluminum cylinders made of 6351 alloy. Eleven of the failures occurred when the cylinders were being filled. Six of the failures were SCBA cylinders used in life-support systems for firefighters, and three were SCUBA cylinders used by divers. Although some of the failures were variously attributed to mechanical damage and over-pressurization, in six cases the cause for the cylinder failures was attributed to a phenomenon known as Sustained Load Cracking (SLC).