Mystery as "amazing" mother of quadruplets dies unexpectedly at the age of 36.

Mystery surrounds the death of a US mother of four who died unexpectedly at age 36.

Christina “Tina” Moore passed away suddenly on Tuesday last week, leaving behind her husband Aaron and her eight-year-old quadruplets.

Her heartbroken family and friends are now awaiting autopsy results to determine her cause of death.

"It was a shock," her husband of 14 years, Aaron, told the Omaha World-Herald.

"I miss her so much, but I have to keep coming back to the time we had together. I'm grateful to have had her to teach me what it means to be an incredible parent," he said.

"She was an amazing mum."

He said his wife's proudest role was being a mother to her quadruplets - Jack, Joseph, Noah and Grace.

"I want [my children] to remember an amazingly strong person who lived with faith," he said.

"I want them to know that the best day in her life was the day they were born and that her proudest role was to be their mother."

The Omaha World-Herald has documented the lives of the quadruplets since they were born in January 2009.

When her children celebrated their first birthday, Tina offered some parenting advice to a nearby reporter.

"Cherish every moment you have with them. Hold them," she said.

"If there's a choice between doing a chore or holding a baby, then hold the baby."