China Arnold's daughter had high-heat internal burns, but no outside marks, Ken Betz from the county coroner's office in Montgomery, Ohio, said.

"We have some forensic evidence that is consistent with our belief that a microwave oven was used," Mr Betz said of baby Paris Talley's death.

The mother denies the charge, which comes over a year after the girl died.

Paris Talley, who was one month old, was already dead when Ms Arnold took her to hospital in August 2005.

The mother was briefly arrested at the time and later released while the authorities continued investigating.

The investigation proved tricky because "there is not a lot of scientific research and data on the effect of microwaves on human beings," according to Mr Betz.

Ms Arnold's lawyer, Jon Paul Rion, has denied that his client microwaved the baby.

"China - as a mother and a person - was horrified that such an act could occur," Mr Rion told the Associated Press.

Ms Arnold claims that the night before the baby was taken to hospital she had been left for a short time with a babysitter.

Ms Arnold says that neither she nor the baby's father suspected that there was anything wrong with the child until she was found unconscious the following morning.