The number of unaccompanied teens and children in U.S. custody along the U.S.-Mexico border has reached record numbers, forcing children to stay longer in perilously overcrowded border facilities, many of which are similar to jail, multiple sources who reviewed the most recent government data told ABC News.

There are now 4,276 children in custody, up from about 3,400 earlier in the week. It is a 25% increase, which sources tell ABC News is troubling and could lead to the kind of environment last seen during the 2018-2019 surge, in which six migrant children died in U.S. custody.

Overcrowding, measured in pre-pandemic levels, has spiked, various sources who reviewed recent government data told ABC News. Rio Grande Valley is at 363% capacity and all the major Border patrol sectors are at well over 100% capacity.