A grandmother died on a motorway hard shoulder as she returned home from mental health treatment more than 200 miles away.
Peggy Copeman, 81, was being transferred to Norfolk from an out-of-area placement in Somerset when she died on Monday.
Her daughter Maxine Fulcher said Norfolk and Suffolk NHS Foundation Trust "might as well have sent her to the moon and back".
The trust has apologised.
Mrs Copeman, of New Buckenham, Norfolk, was transported by a patient transfer minibus to Taunton's specialist Cygnet Hospital on Friday, as no beds were available nearer to her home.
However, on Monday a bed became available at Norwich's Julian Hospital but during her return journey Mrs Copeland died.
Mrs Fulcher said she was called and told her mother, who had early-stage vascular dementia, had become distressed and stopped breathing on the M11, south of Cambridge.
"I just want to know what happened to my mum and why - why she was travelling and why she didn't make it back to Norfolk," she said.
She added: "They might as well have sent her to the moon and back."
Her husband Nick said: "This should never have happened.
"She should be celebrating Christmas with us. This isn't the way we wanted it to end and we don't want this to happen to other families because this is the devastation that it leaves behind."
Norfolk and Suffolk NHS Foundation Trust said it had sent its condolences to Mrs Copeman's family and was "determined" to find out what happened.
It added "in a perfect health care system" no-one should have to travel that far for treatment.
"We are determined to eliminate out-of-area placements for this kind of situation," Daniel Dalton, the trust's chief medical officer, said.
Cygnet Health Care, which runs the hospital in Taunton, said it would work with the trust during its investigation.