MOUNT CLEMENS, Mich. — Verena Dierkes was just two months out of high school when she left Germany to become an au pair in the outwardly idyllic Michigan home of Tara and Stephen Grant.

But it wasn't long, according to prosecutors, before she began an affair with Stephen Grant that drove him to brutally murder and then dismember his wife.

On Tuesday, nine months since she left the U.S. in the wake of Tara Grant's disappearance, Dierkes returned to Michigan as a reluctant key witness in Stephen Grant's first-degree murder trial.

Glaring at the defendant, the dark-eyed, soft-spoken Dierkes described her brief yet eventful association with the Grants and their two children, Lindsey and Ian, who were 5 and 3 when their mother was killed in February.

The relationship began innocently enough, when Dierkes, who was 19 at the time, moved into the Grant home in August 2006 to care for the children during the week, when their mother was likely to be out of town on business. By February, she was sharing a bed with Stephen Grant, who wooed her with suggestive e-mails and text messages in the week before his wife's death.

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When Grant finally succeeded in convincing Dierkes to submit to him sexually, prosecutors say, he was emboldened the next day to brutally kill and dismember his wife so he could be with his teen lover.

Grant, who faces life in prison if convicted as charged, does not deny that he killed his wife or that he cheated on her with their live-in nanny. On Friday, he pleaded guilty to dismembering Tara Grant's corpse, leaving the sole issue for the jury's consideration as whether he plotted to kill his wife or if he acted in the heat of the moment. The distinction could mean the difference between life in prison without parole and the chance of being released one day.

In the defense opening statement Friday, Grant's lawyer, Gail Pamukov, told jurors that simmering tensions between the couple boiled over the night of Feb. 9, causing Stephen Grant to snap.

The two were fighting over Tara Grant's busy travel schedule, according to Pamukov, when the altercation escalated into violence and ended with Stephen Grant strangling his wife to death.

After hiding his wife's body in his vehicle for two days, he brought her to his family's tool-and-die shop and used a band saw to cut up her limbs.

About three weeks later, authorities found Tara Grant's torso in a plastic container in the couple's garage. The rest of her remains were scattered in a park near the couple's Washington Township home.

But Macomb County prosecutors contend that Grant was little more than a serial philanderer who resented his wife's role as breadwinner, which kept her away from home for weeks at a time, and wanted her out of the picture after finding Dierkes.

Dierkes admitted that it did not take long for her to succumb to Grant's charms during a whirlwind courtship that began at the end of January, when he told her about the discord in his marriage.

In the week before Grant killed his wife, a relationship that began with long conversations into the early morning hours progressed into sexually suggestive e-mails and text messages.

"'For what I'm thinking right now, I'd definitely go to hell,'" Dierkes said, recalling one of his e-mails, as Grant, dressed in an olive suit and brown tie, bowed his head, his face flushed.