Thirty-two research monkeys quarantined at a lab in Nevada were accidentally killed in May due to overheating, officials for Charles River Laboratories Inc. confirmed for the first time on Thursday.

Animal rights activists who said they reported the incident to the U.S. Department of Agriculture last month based on a tip from a whistleblower said the longtail macaques “were literally cooked to death by a heating system failure.”

The company based in Wilmington, Mass., confirmed in a statement that the death of the imported monkeys in a single room of its quarantine facility in Sparks on May 28 was the result of a series of human errors in the operation of the climate control.

The incident affected no other primates and “at no point was the public ever in danger,” Charles River said.

The company said it reported the incident immediately to the USDA and the Centers for Disease Control and Prevention. The monkeys were to be used in the research of drugs.

“This is a terrible and unfortunate tragedy,” the company said. It said that in addition to reporting the accident, Charles River officials recently met with USDA to review preventive measures that were put in place as a result of the accident.

“An extensive internal investigation found that, despite the redundancies that were in place, several human errors occurred that led to this tragedy,” the statement said.

The Reno Gazette-Journal first reported the deaths on a blog on its Web site on Thursday, citing excerpts from a company letter it received in response to an inquiry following a tip from a reader.

Charles River said in its statement that while it immediately contacted USDA and CDC, it did not publicize the event out of concern for the safety of its 515 employees who work at the 120,000-square-foot facility in Sparks and a second 370,000-square-foot facility in neighboring Reno.

“Due to the increased level of extremist activity targeting individuals involved in biomedical research in recent months, our first concern is for our employees and the animals we care for at Charles River,” the company said.

Corporate spokeswoman Amy Cianciaruso in Wilmington and Greg Beattie, executive director for site operations in Sparks, said they could not comment beyond the statement.

Kathy Guillermo of the People for the Ethical Treatment of Animals questioned Charles River’s claim it immediately reported the incident to federal authorities because “USDA did open an inquiry when we filed a complaint” in July.

“Their statement about fears for their own safety is absurd. They have an abysmal record of animal care. As usual, they are more interested in their reputation than in the animals’ safety,” said Guillermo, director of PETA’s Laboratory Investigations Department in Norfolk, Va.

“That monkeys were literally cooked to death by a heating system failure, as a whistleblower alleges, shows that the facility did not even have a simple alarm system in place to alert staff to the malfunction,” she said on Thursday.

Justin Goodman, PETA’s research associate supervisor, notified the USDA of the group’s concerns on July 7 based on an anonymous letter PETA received four days earlier. He attached the letter, which said in part:

“In June 2008, the heating malfunctioned at Charles River Nevada in their monkey building and no alarm went off and no one knew until the next morning. Many many monkeys died and maybe many more.”

Robert Gibbens, Western regional director of animal care for USDA’s Animal and Plant and Health Inspection Service in Fort Collins, Colo., said in a July 7 letter back to Goodman that USDA had given the complaint a tracking number and would “look into your concerns and take appropriate action if necessary.”

A spokesperson for his office said he was not available and referred calls to officials for USDA’s APHIS office in Riverdale, Md., who did not immediately return a call seeking comment.

Coral Amende of Reno Outreach for Animal Rights said she received an e-mail from a whistleblower about the incident last week and contacted PETA. She said Charles River could be found guilty of criminal negligence under the Animal Welfare Act.

“I don’t know for sure because I don’t have the details but it sounds like negligence to me with a series of human errors, not just one human error,” Amende said.

Guillermo said USDA cited Charles River for 22 violations of the act in 2005 alone. Another animal rights group based in Ohio, Stop Animal Exploitation Now, publicized earlier this year 20 violations Charles River reported to federal officials in 2006 and early 2007.

SAEN cited information gained through the Freedom of Information Act that showed the fingers of two monkeys at the Sparks lab had to be amputated after they were caught in the wiring of their cages The tip of the tail of a third monkey was cut and was treated.

Cianciaruso said at the time that the company immediately changed procedures to minimize the chance of a recurrence of the injuries and staff were trained on the new procedures.