The acting DHS inspector general said one senior manager at a detention facility described the situation as a "ticking time bomb" and raised security concerns for agency staff and detainees.
Some detainees had attempted to escape during maintenance work, according to the report.
A search has been underway since Monday evening in the Rio Grande River at the USA border for a missing 2-year-old girl.
The report also featured this image of some of the 51 adult women held in a cell built to hold 40 male juveniles pressing against the cell window at the Fort Brown Border Patrol station in Brownsville, Texas.
Conditions at the centers have been a flashpoint since May when the watchdog warned of similar conditions at facilities in the El Paso, Texas sector, west of the Rio Grande Valley, with migrants held for weeks instead of days, and adults kept in cells with standing room only.
One of the congressmen, U.S. Rep. Joaquin Castro, alleged that a woman told them she was instructed to drink water from a toilet.
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"So this is what happened with the migrant and drinking water from toilet: she wanted water, didn't know how to use the faucet in the cell, and drank from the toilet".
"Border Patrol's custody data indicates that 826 (31%) of the 2,669 children at these facilities had been held longer than the 72 hours generally permitted", she added.
According to the CBP, the Rio Grande has the highest volume of migrants on the southwest border, recording nearly 250,000 apprehensions so far this year - marking a 124% increase from 2018. Jeff Merkley, D-Ore., about his attempts at the time to visit detention facilities at the border and being blocked from doing so by border patrol.
Figure 3 shows overcrowding of families observed by the U.S. Department of Homeland Security OIG on June 11, 2019, at Border Patrol's Weslaco, TX, Station.
"Without Congressional action to address legal and judicial loopholes, families and [unaccompanied A children] will continue to be incentivized by the smuggling organizations to make the risky journey and be encouraged by the likelihood that families will not be detained during their immigration proceedings", a DHS response contained in the report reads. CBP rejected the offer. "This is all part of an effort by the administration on their policy from May to say to the world, 'Do not come to our borders if you're fleeing oppression because we will inflict trauma on you and your children'".
"These posts are completely inappropriate and contrary to the honour and integrity I see - and expect - from our agents day in and day out", Border Patrol chief Carla Provost said.
Another group member posted the now infamous photo of a father and his toddler daughter lying face down in Rio Grande after drowning while attempting to cross the border. Some had no access to showers or hot meals and had little access to clean clothes. Some adults were only receiving bologna sandwiches, causing constipation and in some cases requiring medical attention.
Of 2,669 children detained by the Border Patrol in the region, 826, or 31 per cent, had been held there longer than 72 hours. "So you had these rules that had been in the facility - it's either Department of Homeland Security or Health and Human Services - they are imposing two-week requirements, you have to seek a waiver, they don't want to do it except when they can fly in special minders to control everything you do while you're there". In one photo, women and children appeared to be sleeping on the ground under Mylar blankets.