El Paso, Texas (CNN) US authorities are no longer holding migrants under a border bridge here.

But advocates say some children and families who US Customs and Border Protection detained for days in the fenced-in space were shaken once they were released from custody.

Bruises were visible on toddlers and older children who had to lay on rocks and concrete, said Taylor Levy, legal coordinator at Annunciation House.

The El Paso-based shelter is a frequent stopping point for migrants who've been released from government custody. Migrants who'd been held under the bridge told staff there they'd been held from three to five days and "treated worse than dogs," Levy said.

Photos and videos from the scene appeared to show migrant families sitting and at times sleeping on the gravel, with only thin space blankets between them and the ground.

US authorities have drawn criticism for holding groups of migrants awaiting processing -- including children -- underneath the Paso Del Norte Bridge in El Paso.

"It was horrible there, because we slept under the bridge on top of the gravel," said Bartolo Tadeo Gómez, 25, who spent four nights under the bridge with his 7-year-old son. "There was a lot of dust."

They struggled to stay warm without blankets, he said. And the whole time, Tadeo said, he worried about his son, who seemed sick and wasn't eating.

"I was afraid he would die," Tadeo told CNN.

Last week, US authorities said a rapid influx of migrants crossing the border forced them to use the space under the Paso del Norte border bridge as an emergency measure in the face of a humanitarian crisis. They described a tent set up under the bridge as a "transitional shelter" for migrants who had yet to be processed.

CBP officials say caring for those in custody with dignity and respect is a paramount concern.

Advocates accuse CBP officials of worsening the humanitarian crisis at the border, rather than helping it.

Over the weekend, the American Civil Liberties Union filed a complaint over the practice, calling it "extreme and unprecedented."

And the move also drew sharp criticism from the head of the Border Patrol's union, who called it "flat-out wrong."

Here's a look at the latest developments:

The ACLU is calling for an investigation

The ACLU filed a complaint with Department of Homeland Security and CBP officials over the weekend, calling for the DHS inspector general to investigate what occurred and why.

"The detention of migrants for multiple nights in outdoor detention pens is an unprecedented and extreme violation," the complaint says. "Although CBP has long violated the rights of migrants in its custody, the agency's decision to detain migrants, including children, in caged dirt filled outdoor areas is an escalation of this administration's cruelty. Without immediate attention and oversight, we will continue to risk the lives of those seeking refuge in our country."

Other advocacy groups have raised concerns.

On Sunday, Levy tweeted a photo that she said showed a 5-year-old's chapped hands.

these are the bloody & chapped hands of a 5-yr-old child who spent 3.5 days detained by CBP under the bridge in El Paso, TX this past week. her 6-yr-old still has bruises from sleeping on the gravel. pic.twitter.com/jCIs1iZNjH — Taylor Levy (@taylorklevy) March 31, 2019

Asked about the ACLU's complaint and allegations of children with bruises, a CBP spokesman said the agency is doing everything it can to deal with a humanitarian crisis that only Congress can solve.

"The care of those in our custody is paramount," CBP Assistant Commissioner for Public Affairs Andrew Meehan said in a statement. "The Department of Homeland Security (DHS), US Customs and Border Protection (CBP) and the United States Border Patrol (USBP) are devoted to the care and processing of the individuals in our custody with the utmost dignity and respect."

For days, CBP officials have been defending the decision to hold migrants under the bridge.

In a statement released to CNN El Paso affiliate KFOX, the Border Patrol said:

"The tent that is set up underneath the Paso Del Norte port of entry and adjacent to the Border Patrol's Processing Facility is a transitional shelter. Due to the large volume of apprehensions within the El Paso Station's Area of Responsibility, the agency has undertaken additional measures to facilitate processing. As illegal aliens arrive at the processing facility, they are placed at the 'tent' to await their turn to be processed. This tent serves only as a transitional shelter and is not a temporary housing facility. It was established within the last month."

CBP described the area under the bridge as a "transitional shelter."

The Border Patrol union chief says the practice was 'flat-out wrong'

National Border Patrol Council President Brandon Judd called the decision to detain migrants under the bridge "flat-out wrong."

He referred to the recent death of 7-year-old Jakelin Caal in CBP custody.

"After what happened with Jakelin, you would have thought that this would have never happened again," he told CNN. "You can't do that in my opinion."

"It's one thing to have overcrowding indoors, which is bad enough," he continued, "but outside, in inclement weather, in my opinion that's just wrong."

Jakelin died in December from a bacterial infection known as streptococcal sepsis, two days after she and her father were detained. Advocates have raised questions about her care. The CPB said agents and emergency responders "did everything in their power" to treat the Guatemalan girl before she was hospitalized.

Where are the migrants now?

Some migrants have been released from custody, according to Annunciation House, and are starting to share their stories.

Guatemalan immigrant Bartolo Tadeo Gómez, 25, says US authorities held him and his 7-year-old son for four nights under a border bridge in El Paso.

On Sunday, a CBP official told CNN the migrants held under the bridge were being transferred to another location.

CBP decided to relocate to another location with more space and shelter capability due to the steadily increasing number of migrant apprehensions in the area, the official said.

The ACLU suggested other factors might have inspired officials to take action.

Officials, the organization claimed, decided to move the migrants away from the outdoor detention area ahead of a congressional visit to El Paso.