Lithium ion batteries (LIBs) are among the most promising technologies for electrical energy storage. However, any exposure of LIBs to abnormal operating conditions may lead to rapid self-heating accompanied by ejection of flammable materials, termed thermal runaway. In a multi-cell array or pack, thermal runaway may propagate to neighboring cells and grow into a large-scale fire. In this work, a new experimental setup was developed to investigate this propagation or cascading failure phenomenon using 12-18 cell arrays constructed from lithium cobalt oxide (LCO) cells of 18650 form factor and 2600 mA h nominal electrical capacity. The arrays, consisting of fully charged cells, were mounted in a specially designed wind tunnel, which provided well controlled environmental conditions. Thermal runaway was initiated in one cell using a small electric heater and observed to propagate through the array using temperature sensors attached to individual cells. The propagation process was studied in both nitrogen and air environments to elucidate the impact of flaming combustion. In addition to the cell temperatures, production rates of O-2, total hydrocarbons (THC), CO, CO2 and H-2 were measured, and heats generated in chemical reactions between the battery materials and in flaming combustion were computed. In the nitrogen tests, row-to-row propagation speed (S-P) showed no significant dependence on the size of the array and was estimated to be 0.08 s(-1). When cell arrays were tested in air, S-P increased to 0.7 s(-1) (about 9 times the nitrogen Value) in late stages of cascading failure due to the impact of flaming combustion of ejected materials. Measurements demonstrated that failing LCO cells produced minor mass yields of O-2 and H-2 in addition to relatively large amounts of CO, THC and CO2. The lower flammability limit of the ignitable portion of ejected gaseous products was determined to be 5.79 +/- 0.12 vol.% in air. The chemical heat generation resulting from reactions between battery materials inside and outside the bodies of cells was computed to be 56.6 +/- 2.5 kJ per cell. The total amount of heat released from flaming combustion of ejected battery materials during air tests was found to be 60.1 +/- 17.5 kJ per cell. The efficiency with which these battery materials were combusted was estimated to be about 56%. The results of this study provide previously unavailable, comprehensive assessment of the failure dynamics and energetics in LIB cell arrays or assemblies. These results are expected to serve as a foundation for effective methodologies for detection, mitigation and prevention of electrical energy storage and electric vehicle fires.