The San Bernardino killers' six-month-old daughter could now be adopted by her uncle.
Farhan Khan, who is married to the sister of gunman Syed Rizwan Farook, said his attorneys are already working towards her adoption.
"Hopefully we'll go through the process that we can adopt her. That's the plan," he told NBC News.
Speaking out for the first time, Farook's sister Saira Khan told CBS News that she cannot fathom how they could leave the child behind.
"He put us in this predicament," Saira said. "It's just mind boggling why they would do something like this."
Farook and his wife, Tashfeen Malik, left the little girl with Farook's mother on Wednesday morning, saying that they were going to a doctor's appointment.
When the couple failed to return hours later, the family became concerned. They only started to figure out what had happened when contacted by a reporter, according to Hussam Ayloush, executive director of the Council on American-Islamic Relations.
Farook and Malik had stormed the Inland Regional Center, where a holiday party was underway for the county's department of health employees. The couple was later killed during a shootout with police.
Their child is now in protective custody.
Khan said he is outraged that his brother-in-law left the little girl behind.
"You left your six-month-old daughter in this life," he told NBC. "Some people cannot have kids. God gave you a gift of a daughter. And you left that kid behind. What did you achieve?"
He said the anger over the abandonment "hits me every few hours."
Khan added that he had "no idea" what caused the couple to open fire, killing 14 people and injuring 21 more.
"Did somebody brainwash him?" he asked. "Something snap him? What - did he have - a fight at work? And I'm waiting… to hear, like, you know, what really happened."