Exclusive: Unaccompanied migrant children staying in Border Patrol facilities an average of 107 hours, internal records show

Thousands of migrant children who crossed the US-Mexico border alone are being held in US Border Patrol custody for more than four days on average in facilities unfit for minors, a dramatic increase from a week ago, according to internal documents reviewed by CNN.

The data, dated Tuesday, is the latest indication of the challenge facing the Biden administration to address the bottleneck in the immigration system, with the number of unaccompanied children outpacing the availability of proper shelter space to care for kids.

On average, over the last 21 days, Customs and Border Protection encountered 435 unaccompanied children daily, up from a previous average of around 340 children.

They are staying in facilities for 107 hours on average, longer than the 72 permitted under US law. A week ago, the average time was 77 hours.

This story is breaking and will be updated.