In what could be a major blow to autism research efforts, scientists say that one-third of the world’s largest collection of autism brain samples has been damaged.

Officials at McLean Hospital, a Harvard affiliate near Boston, found that a freezer where the samples were housed failed in late May, according to a report in The Boston Globe. Two alarms designed to warn of freezer problems had not gone off even as the internal temperature rose and all of the 150 brains inside thawed.

Of them, 53 were intended for autism research, according to Autism Speaks, which runs the Autism Tissue Program that the samples are part of. The collection includes brain tissue from individuals with autism who have died.

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Experts told the Boston Globe that the damaged brain samples represented a “priceless collection” and the loss could slow autism research efforts by a decade.

Officials at Autism Speaks, however, were more optimistic.

“Although this event will affect the availability of tissue for future research, we cannot yet determine the level of impact, but we are confident that we can maintain the momentum of scientific studies based on brain tissue,” wrote Geri Dawson, the organization’s chief science officer, in a letter to supporters.

Dawson said that all but one of the damaged brain samples were bisected. As a result, just half of each had been stored in the problematic freezer. The other half of each brain sample remains available for research.

It is not clear what led to the freezer malfunction and a number of investigations are underway. However, given the unusual circumstances, officials at the center where the brain samples were stored told The Boston Globe that they cannot rule out foul play.