At the heart of both groups' protest was a change to U.S. immigration regulations made in June 2012 giving something called deferred action to immigrants who were brought to the country illegally as children. Those who were in the U.S. at that time and met a list of criteria could apply for a renewable two-year deferment and work authorization.
But the young people crossing Monday had left the U.S., either voluntarily or through deportation, months, weeks or even just days before the deferred action announcement, commonly known as DACA.
"We look at this action today and the Dream Nine as a type of extension of DACA," said David Bennion, an immigration lawyer traveling with the group. "What we would like to see is the people who left, like these 30 who otherwise would have qualified for DACA, to have that be taken into consideration."
There were several minors in the group, including 17-year-old Luis Enrique Rivera Lopez. He came to the border from Guasave in Sinaloa, a Mexican state that he had known only by its reputation for drugs and violence before going there from Los Angeles early last year.
"I wanted to have a sense of my roots," Rivera said of his decision to return to Mexico, where he hadn't been since he was 1. "I wanted to know where I was from."
The experience was rewarding in some ways. He got to know both sets of grandparents. But after 19 months away he missed his parents and three siblings who remained in Los Angeles. He also found he didn't fit in after having grown up in Los Angeles.
"When I got to Sinaloa I didn't dress like anyone. My haircut was different. My style of walking was different. My Spanish was like way off," he said.
David Leopold, an immigration lawyer and past president of the American Immigration Lawyers Association, said the tactic concerned him.
"The focus now should be on getting the House of Representatives to do its job and fix the immigration system," Leopold said. "I don't know that these actions move that issue forward."