The study aimed to show that Alzheimer’s disease could be stopped if treatment began before symptoms emerged. The participants were the best candidates that scientists could find: still healthy, but with a rare genetic mutation that guaranteed they would develop dementia.

For five years, on average, the volunteers received monthly infusions or injections of one of two experimental drugs, along with annual blood tests, brain scans, spinal taps and cognitive tests.

Now, the verdict is in: The drugs did nothing to slow or stop cognitive decline in these subjects, dashing the hopes of scientists.

Dr. Randall Bateman, a neurologist at Washington University in St. Louis and principal investigator of the study, said he was “shocked” when he first saw the data: “It was really crushing.”