The “catastrophic” failure of a storage tank this month at an Ohio fertility clinic caused the apparent loss of more than 4,000 frozen embryos and eggs, the clinic said this week.

About 950 patients were affected by the failure, in which the tank’s temperature rose and an alarm did not go off, the University Hospitals Cleveland Medical Center, which oversees the clinic, said in a letter that was sent to patients on Monday and posted on its website.

“We are heartbroken to tell you that it’s unlikely any are viable,” the letter said.

The letter was an update to an announcement on March 8 that the tissue storage bank where eggs and embryos are stored in liquid nitrogen had experienced an unexpected temperature fluctuation the previous weekend. At the time, the hospital said it did not know whether the eggs and embryos were still viable.

In its letter this week, the organization said that the 4,000 specimens affected were at least twice as many as it had originally estimated.