Heidi Broussard, 33, hasn't been seen since she dropped off her 6-year-old son at Cowan Elementary School in Austin early Thursday, before she left with her infant daughter, Margot Carey, who is now 3 weeks old, police said. Investigators believe they went back to their apartment complex before they disappeared, police Detective Brad Herries said.
The FBI assigned its Child Abduction Rapid Deployment team to assist local authorities, Herries said Tuesday.
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"To be perfectly honest with you, this case is unique in that we don't have a person of interest right now," Herries said. "We're exploring every avenue that we have and every possibility."
Margot's father, Shane Carey, who is Broussard's boyfriend, said the couple also have a 6-year-old son together. Carey told NBC affiliate KXAN on Monday that their son's school never called when Broussard didn't show up in the afternoon.
Carey said that he saw Broussard's car when he came home Thursday afternoon but that she wasn't in their apartment. He said he wasn't initially worried because Broussard often visits a friend in the same complex. He called police at 7 p.m.
Carey also said he believed that if Broussard left on her own, she would have told someone. He said all of Broussard's belongings, including her identification and the baby's diaper bag, were still in the apartment.
"If she left, everything is here," Carey told NBC News on Tuesday. "And she would tell her friend if she left, so that's scaring me more."
Herries declined to answer when asked Tuesday whether there had been previous domestic abuse reports from Broussard's apartment. He said authorities had no reason to believe that Margot or her mother had been harmed.
"It's possible that Heidi and Margot have left on their own accord," Herries said. "It's also possible that there may be something nefarious or foul play involved. To be honest, we don't know the answer to that question."
Security cameras showed that Broussard was wearing a purple sweatshirt when she was at Cowan Elementary. Broussard is 5 feet, 3 inches tall, weighs 150 pounds and has long, dark hair with highlights.
Police asked the public to continue to call in tips with any information on Broussard's and Margot's whereabouts over the last five days.
Doha Madani is a breaking news reporter for NBC News.