Michigan: Delta Air Lines is investigating the death of an eight-year-old pet dog during a stopover at Detroit Metropolitan Airport, en route from Phoenix to Newark, New Jersey.

The Pomeranian, called Alejandro, was found dead on Wednesday morning in its carrier in a cargo facility at the airport, south-west of Detroit in Romulus.

"When he landed here in Michigan, he was alive at 6.30am, and then at 8.20, he wasn't moving and it just doesn't make any sense to me," owner Michael Dellagrazie told WDIV-TV.

"We lost a family member. That's exactly what happened, and somebody has to be responsible for it. He was in their care and they didn't take care of him."

Delta told WXYZ-TV that a flight attendant checked on Alejandro about 6am. The attendant checked again about two hours later and the dog was dead.