(CNN) In the letter, she calls the facility "la perrera" -- the kennel or dog pound -- because of the chain-link cages she and others were held in.
For eight days after she was captured near the US-Mexico border, she says she did not bathe or brush her teeth. She and other women slept on the floor under "aluminum paper" blankets, she says.
"They treated us so horribly, as though we were animals," she wrote in the letter, in which she called herself "anonimo," anonymous.
But worst of all was not knowing the fate of her daughter or son for 21 days after they were taken from her, she wrote.
The account came via Grassroots Leadership, a nonprofit that describes itself as "a nationally recognized civil and human rights organization" that fights to "end prison profiteering, mass incarceration, and deportation." The group, which is circulating a petition to reunite separated families, posted the letter on its website on June 25. Four days later, the group posted several more accounts from women who had been separated from their children.
Grassroots Leadership volunteers met with the women in T. Don Hutto Residential Center in Taylor, Texas, organizer Bethany N. Carson told CNN. Those volunteers spoke to the women about writing the letters and "helped them figure out how to get the letters out," she said in an email.
"They were written to tell their experience publicly to ask for help in being released from detention and reunited with their children," Carson said.
CNN reached out to US Immigration and Customs Enforcement, which oversees Hutto and other facilities where migrants are held while they await legal proceedings, for comment on allegations of mistreatment made in the letters. But the letter writers' names, when provided, are redacted, and it's not clear which facility each letter describes. Without knowing a facility's name or the person's identity, an ICE spokesperson said the agency was unable to address the specific claims.
A lawyer for the anonymous letter writer said her account describes a holding center near Hidalgo, Texas. The woman stayed there before she was transferred to another center in Laredo, and then finally Hutto. But even her lawyer said she did not know the exact locations of those holding centers.
Some of the accounts offer a glimpse into conditions in the facilities. Other women simply used the letters to lay bare their heartache and offer words of comfort to their children.
One woman, Mirian, wrote:
"My son, I write these words with an immense pain in my heart for having you separated from me. But I want you to know that I miss you a lot. And that every day, I pray to God that we'll be together again soon and that they will never again separate you from me because you are the most beautiful thing that God has given me, you my son. I want you to know that I love you with all my heart and what I am asking God the most is that we will be together."
In a separate letter, another woman from Guatemala addressed her 5-year-old son, a spokesman for her lawyer said. Her husband was already in the United States and took custody of the boy. The woman has spoken to her husband, but she and her son are "too emotional" to speak to each other, Chris Lippincott said.
"I hope to see you soon. I know you're with your father but soon the three of us will be together. I know you miss me and I miss you too and when we're together, we'll go out and have fun. I don't want you to be sad. I love you my son. See you soon. I send you hugs and kisses."
Other women addressed a broader audience in their letters.
One woman who gives her name as Claudia says her story began on May 21, when she and her son crossed the river that is the US border. Two days later, she says, she was separated from him with "lies" when she was brought to criminal court without realizing it.
"I cried a lot and I felt like I was going crazy, like part of my life was missing and I wasn't complete," she wrote in a letter dated June 28.
"People come here to seek asylum, not to be imprisoned like a criminal or to have your child taken from you. In all this time, we've only spoken three times and the last time he told me he was sad and asked when we would be together again. And that tears my soul apart. We want justice and to be reunited with our children as soon as possible. We are human beings and we are spending many nights suffering."
The anonymous letter writer came from Guatemela with her 9-year-old daughter and 17-year-old son, her lawyer, Stephanie Taylor, said. Her husband was already in the United States pursuing an asylum claim, and the family wished to be reunited, she said.
Taylor confirmed the woman's account that she spent eight days in "la perrera," the first place she ended up with her children after they were captured in May near Hidalgo, Texas. She was separated from her son as soon as she arrived at the facility, and her daughter stayed with her for one day before she, too, was taken. On May 19, the children were taken to another location in Texas before landing in a center in Michigan, Taylor said.
The woman describes the anguish of the separation in her letter.
"From then on, I didn't know anything more about my children, I asked them and they told me they didn't know anything. I asked them for a minute to speak with my husband and they didn't let me, all the mothers were crying in anguish, distraught from not knowing anything about our children, this is the harshest thing they could do, to take our children from us. They told us our kids would be adopted by other people."
At the first holding center, some women were allowed to stay with their infants, she said. To them she attributed a different kind of suffering.
"... They cry from hunger and cold, it makes you sad to see these tiny, defenseless babies," she wrote. "The pregnant women faint, and also the women who cannot stand the pain of not knowing (anything) about their children, their whereabouts..."
The woman says she finally heard her children's voices again 21 days after she was separated from them. The children were released to their father on June 19, the lawyer said.
From the first holding center she went to another one in Laredo, and then to Hutto, transported with her feet and wrists in chains, the letter says. She was released from custody June 26.
"This is an account of what I have lived, thank you for your attention."