Two small, quick-moving brush fires that started burning on land in the Rocky Mountain Arsenal National Wildlife Refuge located in Commerce City were human-caused, according to staff at the refuge on Thursday. Crews responded to the brush fires and asked surrounding fire departments in South Adams County and Brighton to support firefighting efforts. The refuge was closed during the response. Rocky Mountain Arsenal posted on social media just after 8:20 p.m. that both fires were fully contained and that they were human-caused. It's not clear exactly how the fires started. It was the second fire at the refuge in less than a week. A fire on Saturday grew to 12-15 acres before its spread was halted. A Rocky Mountain Arsenal spokesperson said it was started by a cigarette. The 15,988-acre wildlife refuge is located about eight miles northeast of downtown Denver. It's home to a herd of more than two dozen bison, along with deer, raptors, songbirds, waterfowl, prairie dogs and coyotes.