Two baby bear cubs unexpectedly die at Rhinelander animal shelter

Two adopted bear cubs have passed away at wildlife rehab center in Rhinelander on Monday.

Wild Instincts said on Facebook that the two cubs had been discovered alone inside a den amid freezing temperatures in late January.

That's when the shelter stepped in to raise the abandoned bear cubs.

But the facility provided a heartbreaking update on Monday, writing that the cubs had passed away "suddenly and unexpectedly" over the weekend. They had been healthy up until that point.

"We take every loss personally, but this has rocked us to our core. In 30 years of rehabbing and 375 bears, this is the very first time ever we have lost eyes-closed cubs," according to Wild Instincts.

The facility says they have several high-tech ICU units, that control the temperature and humidity in the area where the cubs lived.

The cubs were slowly being weened off of the incubator area, with plans to be moved out to the next stage of rehab the every next day, according to Wild Instincts.

But just after noon on Sunday, a heating element in the ICU suddenly malfunctioned, enough to melt some of its housing - but not enough to set off alarms.

"This resulted in the cubs succumbing to toxic fumes before anyone could bat an eye, sort of like an electrical house fire of their very small house," according to Wild Instincts.

"We are devastated."