NEW YORK -- The "mystery witness" in the Nixzmary Brown case has testified that the girl's mother said that she took part in killing the 7-year-old after discovering the girl having oral sex with her stepfather.

The witness had spent time in jail with Nixzmary's mother, Nixzaliz Santiago, and based her testimony on conversations the two women had when they were prisoners together.

Transcript: Mystery Witness Testimony

The witness said Santiago said she wanted to hurt Nixzmary because she had caught her daughter having oral sex with her husband, Cesar Rodriguez, several times.

According to the transcript, the witness testified:

"[Santiago] told me her child was a bad kid and her and her husband killed her."

"[Santiago] dreamt that they were beating the child in the bathtub. That they were using his fists to beat her up."

"And, he walked away and she stayed, keeps going and Nixzmary became lifeless."

"[Santiago] wanted to hurt her because she caught her with her husband several times having oral sex."

On Thursday, Nixzmary's grandmother said the 7-year-old lived in fear of the stepfather charged with murder in her death.

Grandmother Maria Gonzalez also testified that she saw Cesar Rodriguez shake Nixzmary "really hard" two months before the malnourished girl was fatally beaten in January 2006.

Gonzalez was testifying for the defense at Rodriguez' trial. But she said Nixzmary and her siblings respected Rodriguez "out of fear" and ran to their rooms when he got home.

Defense lawyer Jeffrey Schwartz noted that the grandmother testified in Family Court last year that she saw no indications of trouble when she visited the family in November 2005.

Schwartz has portrayed Nixzmary's mother as the killer. Santiago faces a separate murder trial.