More than three weeks after a ""catastrophic failure" at its fertility clinic in Ohio, the University Hospitals Fertility Center is now saying 4,000 frozen eggs and embryos were lost — twice the number the hospital initially reported.

Nearly 1,000 patients of the University Hospitals Fertility Center in Cleveland were sent letters Tuesday apologizing once more and acknowledging some of the reasons for the failure to preserve frozen eggs and embryos held in a storage tank. The hospital is now blaming human error for the devastating loss.

In the letter from University Hospitals Fertility Center, patients received the crushing news that it’s unlikely that any of the frozen eggs and embryos are viable. The letter also confirms that the remote alarm system on the tank — which should have alerted an employee to temperature swings — was shut off during the weekend of March 3. It was during that time the temperature in the tank began to rise, destroying the thousands of frozen eggs and embryos.

"We don’t know who turned off the remote alarm nor do we know how long it was off, but it appears to have been off for a period of time," the letter states. "We are still seeking those answers."

At the same time, NBC News has uncovered a history of problems in other tanks from the same manufacturer.

In crushing news for the affected patients, it’s unlikely, the hospital says in its new letter, that any of the stored eggs and embryos are viable.

In the letter, University Hospitals Fertility Center says the remote alarm system on the tank — which should have alerted an employee to temperature swings — was shut off. The center does not know who turned it off, but it was off during the weekend of March 3.

Despite 11 miscarriages, Wendy and Rick Penniman had two healthy children with the help of University Hospitals Fertility Center. But now, there’s no hope of using their frozen embryos to have a third child.

“It's devastating, I think, for us, but it's more devastating because it's taking something away from our kids,” said Wendy Penniman, 41.

Christina and Marc Ellis wanted to use their two remaining embryos at the center to give their daughter, now 2, a sibling.

"I don't know if I can go through the whole process again. And what's the outcome gonna be?" Christina Ellis told NBC News.

Wendy and Rick Penniman and their children, Beau and Molly Kate. After Wendy suffered multiple miscarriages, the Pennimans' two children were born with the help of the University Hospitals fertility clinic. The couple hoped to have a third child, but have been told their two remaining frozen embryos are no longer viable. Courtesy Wendy Penniman

Both couples are now suing. So are at least 16 other sets of plaintiffs, all claiming negligence and breach of contract.

History of malfunctions

And there may be an underlying technical problem — one that’s happened before.

An NBC News investigation has found the manufacturer of this particular storage tank, Custom Biogenic Systems, has a history of previous malfunctions dating back almost 15 years.

In 2003, Rachel Southwood’s husband Andrew had sperm frozen in the UK after he was diagnosed with cancer. The Tauton, England, couple had one child and hoped they would be able to have another.

“He gave the sample and as far as we were aware, we would potentially be able to have more children,” Southwood said.

But less than a year later, the freezer storing his sperm malfunctioned, rendering the sperm no longer viable. Andrew Southwood eventually died from cancer.

“They simply said that there had been a technical malfunction with the unit that was storing the sperm of my husband,” said Southwood, who was 30 when the malfunction occurred.

In 2003, British regulators warned that freezers made by CBS, a Michigan company, had problems with their automatic filling mechanisms.

“The manufacturer is aware of 21 incidents” in the UK, regulators said.

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“The alert was withdrawn,” according to a statement from the UK’s Medicines and Healthcare Products Regulatory Agency to NBC News. “The UK supplier informed MHRA that 95% of the devices in the UK had been returned and upgraded. The other 5% were to be complete by 2006. Therefore, this issue should no longer be affecting the UK. No further reports have been received by MHRA since September 2005.”

But two years later, after the warning was first issued, a similar incident occurred in Gainesville, Florida. Up to 60 male patients, many with cancer, lost stored sperm when a tank made by the same manufacturer failed.

Experts estimate that there are around 500 clinics that specialize in assisted reproduction around the country, based on a 2015 report about the state of assisted reproduction by the Centers for Disease Control and Prevention. That report included a list of 451 clinics that have provided them with data and 35 others that had not.

'Almost no regulation'

The lack of reliable data on procedures and clinics in the United States — and possible problems with their frozen eggs and embryos — has contributed to a “regulatory vacuum,” says Dov Fox, director of the Center for Health Law Policy and Bioethics at the University of San Diego.

“There is almost no regulation or oversight of any kind that relates directly to the prevention of mistakes like these,” said Fox, who is working with one of the plaintiffs in the Cleveland case.

“And it's not just fertility clinics, it's sperm banks, egg vendors, surrogacy agencies, in all these areas, we don't know how common the mistakes are, because there is no reporting, no tracking agencies.”

NBC News contacted federal regulatory agencies and and organizations that provide accreditation for fertility labs. When NBC News’ Investigative Unit asked who’s ultimately responsible, most said it wasn’t them.

The only government agency that told NBC News it technically could regulate storage tanks is the Food and Drug Administration, or FDA. But because freezers in fertility clinics have not been specifically marketed as medical devices, it doesn’t.

Members of the assisted reproduction industry say tank failures like the one in Ohio wouldn’t have been prevented with more regulations.

“Autofill tanks still require regular periodic checks by laboratory staff to ensure that they are functioning properly,” said Dr. Alan Penzias, a fertility specialist at Boston IVF, an associate professor of obstetrics, gynecology and reproductive biology at Harvard Medical School, and chair of the practices committee at the American Society for Reproductive Medicine, or ASRM.

“I spoke with members of my laboratory staff who informed me that this is common knowledge.”

Fear of lawyers may have played a role in stopping wider reporting of previous failures in the US, says Dr. Mark Jutras, lab and medical director at Advanced Reproductive Concepts in Charlotte, NC.

“It is usually institutions who have failures and their lawyers will not allow outside people to analyze the situation to find the root cause of the problem and how it could have been avoided," said Jutras.

That’s little comfort for the patients who lost their frozen eggs and embryos at University Hospitals Fertility Center.

“Who else has the tanks,” asked Wendy Penniman. “And how many babies are at risk right now while we sit, while we talk?”

The University Hospitals letter told patients the facility had obtained another working storage tank and had planned to move frozen eggs and embryos into that extra tank before the weekend of March 3 but hadn't started that process when the failure occurred.

The fertility center has offered to pay for treatments for any patients who want them.

The Ellises are considering that. The Pennimans declined the offer.

CBS provided NBC News the following statement:

"Custom Biogenic Systems (CBS) is aware of the incident at University Hospitals Ahuja Center in Beachwood, Ohio and is currently in the process of gathering information.

Our objective is to work diligently to support the investigation into what occurred and be helpful in the search for answers in this difficult situation. We continue to have confidence in our products and our people that serve our customers."

CBS sent a second statement to NBC News on Tuesday evening. In it, CBS denied that issues with the auto-filling mechanism constituted a “malfunction” or an underlying “technical problem.”

Dr. James Liu, chair of the department of Obstetrics and Gynecology at University Hospitals told NBC News Tuesday that the fertility center did contact CBS when problems with the tank were discovered several weeks ago.

“They did tell us that this is how we would do preventative maintenance on that particular problem, and that we recognize the problem, and that they were aware of that,” said Liu.

In its letter to patients, the University Hospitals said preventative maintenance was needed because the auto-fill system had "difficulty" and was "not working."

In its second statement, CBS questioned the hospital's method for filling the tank with liquid nitrogen during this period.

“The CBS tank is not designed to be filled by liquid nitrogen being poured into the top of the tank.”

In addition, CBS noted something which the hospital conceded in its letter — that the remote alarm system was off. Further, the company points out, "CBS did not design, manufacture, install or monitor any remote alarm system on the unit."

In a third emailed statement to NBC News, CBS stated its position that the alert issued in 2003 and later withdrawn by regulators in the United Kingdom “involved a prior design of the unit which was discontinued in 2003.”

In that same statement, CBS also alleged that the incident in Florida in 2005 “was caused by human error” — contrary to media reporting at the time, which was affirmed by a University of Florida Health spokesperson in an email to NBC News last week.

CBS’ third statement to NBC News also included additional information about a “local” alarm relating to the temperature in the tank at the time of the incident.

“When the temperature began to rise on Saturday night,” the statement said, “the CBS unit functioned properly by indicating a high-temperature condition and activating a local alarm. CBS is not responsible for the alert not being sent to the UH employee or for staffing of the UH lab, both as referenced in the UH letter.”

In a user’s manual obtained by the NBC News Medical Unit, the manufacturer devotes a whole page to “Recommended Best Practices in the industry for safe sample storage.”

In a section titled, “Why do I need a Secondary or Back-Up Alarm?” the manufacturer also warns, “Your freezer could malfunction. By having a secondary or back-up alarm you can minimize the possibility of a loss due to a malfunction.”

Editor’s Note from 3/26/18: This story has been updated to include a statement from Custom Biogenic Systems. The statement was sent to NBC News on Monday via email, but appeared to be spam and was not opened until Tuesday.

Editor’s Note from 03/29/18: CBS emailed NBC News a third statement Wednesday evening, which included additional information now added to this report.

Courtney McGee, Jane Weaver, Susan Wagner and contributor Linda Carroll contributed to this NBC News report