The findings showed infants were kept apart from their parents for up to half a year and at least 241 separated children were kept in Border Patrol facilities for longer than the 72 hours permitted by law.
In other cases, it has emerged many separated children were kept in government custody for longer than previously known, with at least 679 children held for 46 to 75 days and more than 25 held for more than a year.
New information about at least 2,648 children who were separated from their parents has been included in the report, which largely covers cases between April 2018 and 26 June 2018, when the Trump administration’s “zero tolerance” policy was in place.
The committee has alleged the policy of separating children “appears to be a deliberate, unnecessary and cruel choice” by the Trump administration.
It argued the policy was unnecessary based on the rationale provided by the administration, who have claimed separations were needed to criminally prosecute parents.
However, documents seen by the committee described parents who were never sent to federal criminal custody and some who were only briefly taken into custody.
Last year, a federal judge in California ordered the government to reunify many migrant families who had been separated at the border as a result of the “zero tolerance” policy.
At one station in El Paso, the politicians said they saw women packed into stuffed cells without running water.
“It was just palpable walking into that cell, you just felt suffering and trauma and fear,” representative Ayanna Pressley said on Thursday, ahead of her testimony.
The hearing comes as surging numbers of families, children and other migrants entering the US from Mexico have overwhelmed the government’s ability to house them adequately.
Democrats have accused Mr Trump of tolerating badly overcrowded and unpleasant facilities in an attempt to discourage future immigrants.
Mr Trump has claimed migrants who have fled Central American countries are “living far better” in detention centres than they would be at home and said the stations are “run beautifully”, despite evidence to the contrary from people who have visited them.
“The situation is worse than they claim it is,” Elijah Cummings, chairman of the Oversight committee, said on Thursday.
“Are you [the Trump administration] going to sit there and say we should be blind to what we see?” he added.
A report last week by the Department of Homeland Security‘s inspector general found “serious overcrowding and prolonged detention” of children, families and single adults at border facilities in Texas’ Rio Grande Valley, the sector with the highest number of apprehended migrants in the country.
Agencies contributed to this report