Torrential rains in central Mexico suddenly flooded a hospital early Tuesday, killing 16 patients, possibly due to the loss of oxygen equipment as the power went out, the national Social Security Institute said.

A video posted on the agency’s social media feed said about 40 other patients survived as waters rose swiftly in downtown Tula, about 60 miles (100 kilometers) north of Mexico City, and flooded the public hospital around 6 a.m.

Video recorded inside the hospital showed knee-deep water as staff frantically tried to move patients.

Later Tuesday, emergency personnel evacuated the hospital, loading patients into ambulances to be taken to other health centers.