MEXICO CITY — Earlier this month, 29 migrant parents from across Central America who were separated from their children last year by U.S. immigration officials returned to the U.S. border.

They asked to be allowed back into the United States to resume their asylum applications and to be reunited with their children, who remained in American foster homes, shelters or with relatives. After about 10 hours of waiting on March 2, the parents were escorted into Calexico, Calif., by U.S. Customs and Border Protection officers, as a group of immigration attorneys and advocates applauded.

But nearly a month later, 17 of those parents are still in the custody of Immigration and Customs Enforcement. Their detention has drawn the attention of Democratic members of Congress, who this week called for their release. Four U.S. senators, Catherine Cortez Masto (D-Nev.), Kamala D. Harris (D-Calif.), Richard Blumenthal (D-Conn.) and Jeff Merkley (D-Ore.), wrote a letter to Department of Homeland Security Secretary Kirstjen Nielsen about the cases.

“It is imperative that these parents be released, reunited with their children, and allowed to continue their cases in immigration court,” the senators wrote. “These families have already suffered significant trauma at the hands of the U.S. government. Given the grave risks to the health and wellbeing of these children and families, we urge you not to prolong their separation.”

The 17 parents were separated from their children under a Trump administration immigration enforcement policy last year. More than 2,700 children were separated from their families along the border last year, according to the Department of Health and Human Services. About 430 of the parents were deported without their children.

Once those parents returned to Central America without their children, some chose to give their children a chance at asylum in the United States, agreeing to let their kids remain with sponsors. Some hoped they would eventually be allowed back into the United States to be with their children.

The American Civil Liberties Union filed a lawsuit asking the government to allow 52 parents back into the United States to pursue their asylum claims. Lawyers said those claims had been blocked after the parents were separated from their children at the border. But so far that case has not been resolved.

The 29 parents, who were assisted by the legal organization Al Otro Lado, decided their best chance at seeing their children again was showing up at the border and pleading with U.S. officials. But after being escorted to Calexico, only 12 parents — those who had come with other children — were released and told when to appear in court. The other 17 are still detained at Imperial Regional Detention Center in Calexico.

“U.S. Immigration and Customs Enforcement (ICE) is working with the Department of Justice to review these cases. In the meantime, ICE is facilitating communications between family members once the relationships are verified,” said Paige D. Hughes, an ICE spokeswoman.

Lawyers for the parents say the detention has been particularly taxing on the parents.

“Their initial detention and separation was incredibly traumatizing. They knew nothing about their children the whole time they were detained. Some have suffered from severe mental health issues since then,” said Erika Pinheiro, a lawyer for Al Otro Lado. “Now they’re back in the same physical space, suffering again.”

Pinheiro said that several of her parents have called her crying and that some exhibit “all the hallmarks of PTSD,” or post-traumatic stress disorder.

It’s not uncommon for asylum applicants, particularly those traveling alone, to remain in detention until they pass an interview demonstrating that they have a credible fear of returning to their home country, the first major step in their asylum applications. Many are detained well beyond that, often for the duration of their asylum cases.

But lawyers, activists and some members of Congress had hoped that these parents would be promptly granted parole.

“Given what these parents have been through, we would have hoped the government would have reunited them with their children while they wait for their hearings, or at least given them hearings promptly,” said Lee Gelernt, the lawyer handling the ACLU case. “This is yet another instance of the government refusing to ameliorate the harm done by their unconstitutional and cruel family separation policy.”