Oregon Zoo officials released a report today saying "we will never know" what killed six endangered monkeys shipped to the zoo last month from Harvard Medical School.

The summary report from zoo veterinarian Tim Storms comes five days after WW reported that postmortem examinations of the six cotton-top tamarins showed they probably died of shock.

Nine tamarins were shipped from Harvard to the Oregon Zoo, traveling more than 50 hours in a van. Six of them died two days after arriving in zoo quarantine.

The summary confirms the finding of "stress-related changes in muscle or adrenal gland tissue." But Storms says stress might not be the only reason why they died.

"Although stress responses were a common finding," Storms writes, "the pattern and timing of these deaths does not appear explainable by stress response alone, and it is very likely that the cause of death had many factors.

"Unfortunately, as much as we would like we will never know the specific cause of death in these tamarins."

Storms' report emphasizes that the zoo's veterinary staff gave the monkeys plenty of room to roam in the $8.8 million veterinary medical clinic.

WW first reported June 13 that the tamarins were given nest boxes made out of plastic picnic coolers. That's where the six monkeys were found dead.

The use of coolers as nests has been the subject of debate between zoo staff in the wake of the tamarins' deaths: One keeper wrote to Harvard that "off-gassing" from the coolers might have been a factor in the fatalities.

Storms does not mention the coolers, but says the monkeys were free to enter and exit at will.

"The tamarins were provided with roomy caging with an elaborate array of perches," he writes. "They were provided multiple nest boxes to enter and exit voluntarily as they chose, in order to allow them to comfortably rest."

The deaths of the monkeys came a month after Metro, the regional government that oversees the zoo, fired the zoo director and chief veterinarian.

As WW reported this morning in Murmurs, elected officials at Metro are mostly declining comment on the zoo's troubles. 

The zoo today released photos and video of the three surviving tamarins, which it says are thriving.