A Superior, Wis., couple found dead on the deck of their rural home last week died of hypothermia after getting locked outside, authorities said Wednesday.

A utility worker stopping by to do a service check found the bodies of Richard Ellison, 82, and Shirley Johnson, 70, on their deck around 10:30 a.m. on Dec. 21, said Superior Police Cpt. Thomas Champaigne. A preliminary autopsy determined that Ellison and Johnson died as a result of prolonged exposure.

It's unclear how long the couple had been outside. Ellison and Johnson were last seen alive while running errands on the previous Sunday, when temperatures dipped into the minus 20s, police said.

The couple's home is located in a remote southwestern corner of the northwestern Wisconsin town. Their closest neighbor was half a block away, Champaigne said, with few other people living in the area.

It appears the couple may have misplaced their keys and then been unable to get back inside. They didn't seek shelter at the neighbor's home or in their car, which was nearby, and died sometime after searching through their own belongings. Authorities were uncertain whether they had a cellphone to call for help.

"Lots of things that could have been done that weren't done ultimately added up into a tragic situation," said Champaigne, who said Superior rarely sees exposure deaths.

In Minnesota, two elderly men have died from exposure-related complications this month after wandering away from a senior-care facility late at night. One of the men, who suffered from dementia, left the Plymouth facility and fell outside.

Several Minnesota college students have also died in recent years from the bitter cold — found frozen in fields, under a bridge, in an alley and on a porch. In those cases, alcohol and cold weather proved to be a dangerous combination, experts say. In almost half of 2014's 26 cold-related deaths, alcohol intoxication, ethanol intoxication or drug addiction was a factor.

Frostbite is a real concern when temperatures plummet. Some studies show that exposed skin can become damaged after just 20 minutes in subzero weather. Wet clothing or high winds dramatically accelerate that time frame. Extremities, especially fingers and toes, are most vulnerable.

"If you're locked out of your house, especially in really inclement weather, call the police," Champaigne said. "We're going to do whatever we can for you; we're going to make sure you have shelter."

Authorities are awaiting final autopsy and toxicology reports but don't suspect any foul play in the couple's deaths.