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A woman who shared a jail cell with a mother accused of killing her baby by burning her in a microwave oven told a jury Thursday that the mom confessed to the crime, saying the month-old girl "fit right in" the oven.

Linda Williams testified that she developed a sexual relationship with defendant China Arnold when the two were cellmates in the Montgomery County Jail in March and that Arnold confided in her about what happened to her baby.

She said Arnold feared her boyfriend believed he wasn't the father of the child and was going to leave her.

"She said she put the baby into the microwave and started it and left the house," Williams said.

Williams said she asked Arnold how she got the child into the oven.

"She said she fit right in," Williams said.

Sitting at the defense table, the 27-year-old Arnold showed little emotion as her trial got under way in the August 2005 death of Paris Talley at their Dayton home.

Under cross examination by defense attorney Jon Paul Rion, Williams acknowledged that she met with police investigators after the conversation and initially said Arnold told her she didn't know how the baby had died.

Williams, who has since been released from jail, said she didn't tell investigators about Arnold's confession earlier because she had feelings for the mother.

"The evidence is going to show that she did not purposely take the life of her own baby," Rion said in his opening statement.

Rion said other people had access to the baby, Arnold was intoxicated to the point of blacking out when the child died and people questioned about the case changed their stories. Rion also raised questions about the reliability of the science when it comes to determining the effect of microwaves on humans.

Russell Uptegrove, a forensic pathologist with the Montgomery County coroner's office, said the thermal burns on the baby were different from those that would have been suffered in a fire, an electrical shock or by hot water, an iron or with chemicals. He said it took him awhile to consider that the burns may have come from a microwave oven.

"It was so heinous to think of that, that I couldn't convince myself it was a real possibility," Uptegrove said.

He said DNA recovered from the ceiling of the oven matched that of the baby.

During the opening statement by Assistant Montgomery County Prosecutor Daniel Brandt, a photo of the burned baby was flashed on a screen for the 12-member jury to see. Arnold sat quietly, occasionally jotting notes on a yellow legal pad.

Brandt said Arnold killed the child after arguing with her boyfriend over whether they had been faithful to each other.

When the couple brought the baby to the hospital, Brandt said, Arnold exclaimed: "'I killed my baby. I killed my baby."'

Brandt said Arnold later told police it never would have happened had she not gotten so drunk. He said Arnold, who has been in jail since she was charged in November 2006, told Williams she had killed the baby in the microwave and other inmates that she hadn't meant to do it.

Rion said Arnold, who has three sons, loved having a daughter and quit college and her job so she could stay home and take care of her. Rion said Arnold's boyfriend was the father of the child, and it couldn't have been anyone else.

During a pretrial hearing on Jan. 11, defense witness Robert Belloto, a staff pharmacist at Good Samaritan Hospital in Dayton, testified that he doesn't believe it would have been possible for Arnold to place the baby in the microwave because the woman was so drunk.

Belloto said Arnold told him she had consumed about 40 percent of a pint of high-proof rum in 90 minutes. But he acknowledged that he had no corroboration for her claim.