Reynosa, Mexico(CNN) The mother's mind raced as she watched her sons from the Mexican side of the Rio Grande.
The two boys were crying and holding hands as they crossed the border alone.
After all they'd been through together, it was shocking to see the boys -- aged 12 and 16 -- slip out of sight so quickly. With just a wave, they were gone. And she was left wondering if they'd made a terrible mistake.
"I felt like I was dying," she told CNN. "I didn't want to separate from them."
But her family felt splitting up was their only option, she says, after they tried to cross into the United States and got kicked out -- twice.
We met this 31-year-old Salvadoran mother when we recently visited a shelter for deported migrants in the Mexican border city of Reynosa. She shared her story but asked not to be identified out of fear for her family's safety. While she waits for word on her older sons' fate, she's taken refuge in this shelter with her younger son -- a 7-year-old with special needs. She isn't sure what they'll do next.
Her story highlights a troubling trend at the border that advocates have criticized as another kind of family separation fueled by US government policies.
A top Border Patrol official told CNN last week that more than 400 children who were taken into US custody as "unaccompanied minors" in south Texas had previously tried to cross with their families. Chief Patrol Agent Brian Hastings, who leads the busiest Border Patrol sector in the United States, says it's a phenomenon his agents are seeing more and more.
We saw this happening during the Trump administration, too, after US policies made it harder for families to cross together. Now advocates warn that once again desperate parents and other adult family members are choosing to send children across the border alone, as US officials expel more migrant families to Mexico under pandemic travel restrictions.
"This comes with great sacrifice. I don't think it's lost on any of these parents," Hope Border Institute Deputy Director Marisa Limón Garza told CNN last month. "This is a grim choice."
How do families make such devastating decisions?
Here's what this mom we met told us about how -- and why -- her family ended up on opposite sides of the Rio Grande.
It's been more than a month since her family fled El Salvador, where she says her oldest son was beaten up when he refused to join a gang and sell drugs.
"We couldn't stay there because of the maras," she says, using a Spanish term commonly used to describe transnational gangs.
What's more, she says, the boys' father left when they were little, and she'd long struggled to make ends meet. Most recently she worked selling vegetables at a market.
"I was earning $5 a day, and that was just enough to pay for food," she says. "I never even had enough to get them a pair of shoes."
Heading to the United States seemed like the best way to save her sons. They made the long journey together. The mother says she never imagined they'd end up apart.
But at the border, she says US authorities sent a clear message when her family tried to cross.
"They said that because of Covid, nobody is allowed in," she says. "I begged them to help me because we can't go back to El Salvador."
Soon, they were expelled to Mexico and found themselves on the banks of the Rio Grande, at a loss for what to do next. That, she says, is when her oldest son made a startling proposal.
"We can't go back to El Salvador. They'll kill us," the 16-year-old said.
Instead, he said, he'd cross the border with his 12-year-old brother, leaving their mom and younger brother behind.
"It's the only way we can get across," he told her.
It wasn't what she wanted, but she knew it was for the best. Weeks later, she still struggles to talk about that moment when she watched her older sons cross the border alone. As she tells us the story, she holds her 7-year-old son tight and wipes away the tears streaming down her face.
"It was the only choice...so they could have a better future," she says.
Days passed with no word from her older children. The mom panicked that they'd been deported back to El Salvador -- that she'd lost them forever that day when they split up.
Then finally, she got an update. Her oldest son called from a shelter in New York.
"They are treating us well. They are giving us food," he told her.
The son tried to reassure his mother and reminded her to take her medicine. Since their family separated, she says she's been feeling ill, and her blood pressure has been shooting up.
"Everything is going to turn out OK," the 16-year-old said.
But their future is far from certain.
The shelter where we met this mother is in a Mexican border state that's notoriously dangerous for migrants.
Just an hour away from here, 16 Guatemalan migrants were killed in a January massacre that made international headlines. Local police have been charged in the slayings.
And the number of families arriving is on the rise.
As political pressure mounted last month amid an influx of migrant children at the border, President Biden said migrant families who've just arrived in the United States should be expelled to Mexico under a pandemic public health order.
"They should all be going back, all be going back," Biden said. "The only people we're not going to let sitting there on the other side of the Rio Grande by themselves with no help are children."
Attorney Jennifer Harbury has been representing migrants in the area for years. She says the Biden administration needs to consider the true impact of these policies.
"People are being hurt, raped, attacked and killed in northern Mexico because we have sent them back," she says. "That's not humanitarian."
A plaza near the border bridge in Reynosa is packed with desperate migrant families -- many who say they were recently expelled from the United States and are unsure of where to turn. This Salvadoran mother we spoke with said she was terrified when she arrived.
"When I saw all the mothers crying in the park, I got scared," she says. Rumors of kidnappings and extortion ran rampant. She knew she needed somewhere safer to go.
She found this shelter and a lawyer who's trying to help her with her case.
And she's turning to her faith to keep her going. She's praying that her older sons will have a better future, and that no one will harm them now that they've made it across the border. She's also praying for what she calls a miracle -- that somehow, she and her younger son will find a way to join them.