“We are there for one another, and I believe that we appeared in each other’s lives at just the right moment,” she said of Mr. Geh, who was taken out of Otay Mesa by ICE on his 29th birthday and is now at a detention center in Alabama.

“I have all of her letters with me,” he said in an interview. “I read them every day.”

The letter-writing project began when the Trump administration’s policy of separating migrant parents from their children at the border was roiling the country last summer. Professor Brooks gathered similarly outraged friends and colleagues at her house; together, they found the names and alien registration numbers for 30 detainees at Otay Mesa who had been part of a migrant caravan held up at the border in Tijuana. The volunteers rented a post office box for the detainees’ responses.

When the first batch of 16 handwritten letters came back July 11, Jennifer Gonzalez, a lawyer in the group, said: “We were all sharing bits and pieces and names and stories. I had this intense sense that each one of those letters didn’t just represent a real person, but they represented a family who missed this person, their community, down to their ancestors.”

Within six weeks, the group had a system in place for writing letters daily and depositing funds into detainees’ commissary accounts. The group estimates it has spent more than $10,000 since July.

“There are 140 of us who have organized to share our commissary,” wrote Ulises, who came to the United States to seek protection from the abuse he suffered as a gay man in Honduras. “We are people from different countries, India, China, Pakistan, Cuba, Jamaica, Vietnam, etc. There are many different languages and sometimes we have to communicate by signals because we don’t speak the same language, but that does not prevent us from providing moral support to each other.”

Terrie Vorono, a volunteer in the group, thanked him for his letter and offered a connection: As the mother of a gay son, she is an advocate for gay rights. “We know that our children are never 100 percent safe even in the United States,” she wrote to him.