LOS ANGELES — Under the blistering Texas sun last July, 37 migrant children boarded vans for what was supposed to be a 30-minute ride. At the end of the road from Harlingen to Los Fresnos lay the promise of hugs, kisses and long overdue reunification with their parents, from whom they were taken when the Trump administration began systematically separating migrant families who crossed the border illegally.

But when the children, all between 5 and 12 years old, arrived at Immigration and Customs Enforcement's adults-only Port Isabel Detention Center, rather than seeing their parents, they saw a parking lot full of vans just like theirs, with children from other facilities who, just like them, were waiting to be processed and reunified with their parents.

It was 2:30 p.m. on Sunday, July 15, 2018.

Not until 39 hours later — after two nights in a van — did the last child step out of a van to be reunited. Most spent at least 23 hours in the vehicles.

After NBC News published its report on the children Monday night, Rep. Greg Walden of Oregon, the top Republican on the House Energy and Commerce Committee, issued a statement calling the children's situation "completely unacceptable" and said he expected a "prompt explanation" from the Department of Health and Human Services and the Department of Homeland Security (DHS).

"To now learn that children as young as five years old were left in vans for more than 24 hours is simply indefensible," said Walden. "I support enforcement of our nation's borders, but ... I strongly believe that children should not be separated from their parents. Period. And those separated should be cared for as if they were our own children until they are reunified with their parents. This is not who we are as Americans."

'There has to be a better process'

It is one of the little-known stories of the chaotic efforts to reunify children following the end of President Donald Trump's "zero tolerance" policy. NBC News has obtained emails sent between employees of BCFS Heath and Human Services, the government contractor and nonprofit organization responsible for transporting the children, who were frustrated by the lack of preparation by ICE, and senior leadership at the U.S. Department of Health and Human Services (HHS).

At 10:30 p.m. local time Sunday, July 15, 2018, Andrew Carter, the BCFS regional director responsible for the children, e-mailed Kevin Dinnin, the company's president and CEO, to alert him to the fact that the 37 children had been waiting for eight hours and not a single one had been processed for reunification.

Click here to see the emails.

"The children were initially taken into the facility, but were then returned to the van as the facility was still working on paperwork," explained Carter. "The children were brought back in later in the evening, but returned to the vans because it was too cold in the facility and they were still not ready to be processed in."