Sen. Kamala Harris (D-Calif.), joined by 16 other Democrats and Sen. Bernie Sanders (I-Vt.), sent a letter to Homeland Security Secretary Kirstjen Nielsen on Tuesday demanding "immediate action" to reunite the immigrant families torn apart and detained under the Trump administration's "zero tolerance" policy.

"Each passing day that DHS fails to act to reunite separated immigrant children with their parents unacceptably exacerbates trauma that this administration has needlessly caused for children and their families seeking humanitarian protection."
—Senate Democrats' letterMore than 500 immigrant children remain separated from their parents—many of whom already have been deported—despite a June federal court order requiring the administration to reunite all families by July 26. Since the administration missed that deadline, immigrant rights groups and critical lawmakers have, as the letter (pdf) puts it, expressed "extreme frustration" over the prolonged separation.

"Each passing day that DHS fails to act to reunite separated immigrant children with their parents unacceptably exacerbates trauma that this administration has needlessly caused for children and their families seeking humanitarian protection," the letter declares, noting that "leading medical experts have condemned family separations as causing children numerous—and in some cases lifelong—psychological and physical developmental harms."

The letter presents Nielsen three solutions to speed up the process:

1. DHS should use humanitarian parole to reunify families with deported parents abroad;
2. DHS should adopt a presumption of reunification for families with parents now deemed "ineligible"; and
3. DHS should ensure that no parents who relinquished their rights to reunification under coercion or duress are removed.

More than 70 percent of the 539 children that remain in government custody, according to the letter, are there because their parents have been deported, and "many such parents are now hiding in their home countries from the very persecutors they fled in coming to the United States to seek protection"—hence the suggestion that DHS offer these parents humanitarian parole.

Another 87 children's parents have been deemed "ineligible" for reunification, yet in many cases, "the government has failed to provide detailed information regarding its allegations." The letter demands that the agency immediately provide such details, and provide parents the opportunity to offer a rebuttal, with the help of an attorney.

The remaining 34 children are in custody because their parents relinquished their right to reunification. The letter states that lawmakers are "alarmed" by allegations which the American Civil Liberties Union (ACLU) has put forth in court that Trump officials misled or coerced these parents to sign such waivers ahead of their deportation.

"In sum, we strongly urge DHS to take all available actions—as quickly as possible—to reunify all families that have been separated for an unacceptably long time as a result of this administration's zero tolerance policy," the letter concludes.

In addition to Harris and Sanders, the letter was also signed by Democratic Sens. Michael Bennet (Colo.), Richard Blumenthal (Conn.), Cory Booker (N.J.), Catherine Cortez Masto (Nev.), Tammy Duckworth (Ill.), Dianne Feinstein (Calif.), Kirsten Gillibrand (N.Y.), Mazie Hirono (Hawaii), Patrick Leahy (Vt.), Robert Menendez (N.J.), Jeff Merkley (Ore.), Bill Nelson (Fla.), Tina Smith (Minn.), Elizabeth Warren (Mass.), and Sheldon Whitehouse (R.I.).