During an exclusive trip to the largest migrant processing center in McAllen, Texas, "CBS Evening News" anchor Norah O'Donnell spoke to Acting Homeland Security Secretary Kevin McAleenan about the crisis at the border. He addressed concerns about conditions at facilities where migrant families are being kept, and the deaths of migrant children in recent months.

"At least four children have died in custody. Are you worried there will be more?" O'Donnell asked.

"I've been worried about that since December. When we lost Jakelin Caal Maquin in early December, that was the first death of a child in our — in our custody in over 10 years, in anywhere in our process," McAleenan said.

According to the Department of Homeland Security, 7-year-old Jakelin and her father were with a group of migrants who turned themselves into authorities along the border in New Mexico in December. Hours after they were taken into custody, she suffered seizures and was airlifted to a children's hospital where she later died.

For more of O'Donnell's report from McAllen and her interview with McAleenan, watch the "CBS Evening News," Wednesday, July 17, 6:30 p.m. ET.