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Tammy Broussard said in an interview airing Friday that she believes the killing of her daughter, Heidi Broussard, whose body was discovered last month in the trunk of a car in Texas, was premeditated.

Magen Fieramusca, 34, Heidi Broussard's former friend, was indicted Tuesday by a grand jury on capital murder and kidnapping charges, the Travis County District Attorney's Office said.

Fieramusca was initially arrested and charged last month with two counts of kidnapping and one count of tampering with a corpse after Heidi Broussard's body was found in the trunk of a car outside a Houston-area residence linked to the suspect. Broussard's newborn daughter, Margo, was found alive in the home.

"Abducted" on "Dateline" airs Friday at 10 p.m. ET

Fieramusca, also known as Maygen Humphrey, was later accused of having faked her own pregnancy in a plan to steal Heidi Broussard's infant daughter, according to an affidavit in support of an arrest warrant.

"She premeditated all of this," Tammy Broussard said in an interview airing Friday on NBC's "Dateline." "We really want to know why. Why would you hurt a friend? I mean, you're supposed to be her friend. Why would you do that? We would just like to know why."

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Broussard said that while she knows her daughter would forgive Fieramusca, she herself has no words. Sometimes, she wants to pick up the phone to call her daughter about something funny, and then she remembers her daughter is gone.

"You know, it's like you see a table full of candles and all lit, but then they go and snuff it out like that," Broussard said. "That's what she did. She took the light away. Heidi was a light in everybody's life."

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Heidi Broussard, 33, was reported missing Dec. 12, and her body was found in a duffel bag in the trunk of a car on Dec.19, the affidavit said. Her friend Rachel West said that she was in contact with Fieramusca during that period and that "she played it off perfectly."

"I talked to Megan every single day, multiple times a day, the whole week Heidi was missing," West said.

NBC News previously reported that Fieramusca told friends that she was pregnant and had an Amazon baby registry listing a due date around the same time as Heidi Broussard's.

Fieramusca told her boyfriend, Christopher Green, that Heidi Broussard's newborn child was her own, according to the affidavit. The day after Heidi Broussard gave birth, Fieramusca told Green that she had gone into labor and had the baby, according to the affidavit.

Fieramusca is being held in an Austin jail, and her attorneys allege that prosecutors have failed to "disclose any evidence" against Fieramusca.

"We call upon patience and resisting the rush to judgment until all the facts are in," Fieramusca's attorneys said in a statement. "Unless the State has proven these allegations beyond any reasonable doubt, Ms. Fieramusca remains innocent."

Tammy Broussard said she had doubts about Fieramusca's alleged pregnancy, saying in her interview with "Dateline" that it didn't look like she had a "pregnant belly." She added that Fieramusca's main concern after Heidi Broussard disappeared was the involvement of police.

"The main concern she had, she was wanting to know are the police doing anything," Tammy Broussard said.

Broussard said she is now focused on ensuring that Margo and her 6-year-old brother know what their mother was like.

"I will keep talking about her and keep her alive in their hearts," she said. "She was the biggest blessing in our life."