An inspection this month at an El Paso Border Patrol station where detained migrants are held found that there were 900 people crammed into the 125-person facility at one point.
The Office of Inspector General for the Department of Homeland Security found "dangerous overcrowding." A majority of detainees were held past the maximum allowed 72-hour period.
Inspectors found migrants held in standing-room-only spaces for days and some standing on toilets.
An unprecedented number of families with children have arrived at the border this year.
The government built a temporary tent city in El Paso and plans to open a bigger facility within 18 months.