From the outside it does not look like much – a tucked-away corner unit opposite a health-food shop in a low-slung, low-rent strip mall in a humdrum suburb on the outskirts of San Antonio.

Despite the modest surrounds, this is the home of a fiercely determined force battling one of the highest-profile and controversial policies of the Trump administration, the separation of families at the southern border.

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Raices, the Refugee and Immigrant Center for Education and Legal Services, has been around since 1986. The not-for-profit group attracted little notice beyond Texas until this year, when almost overnight it became a national focal point for opposition to the Trump administration’s zero-tolerance immigration policy, the subject of a major Guardian investigation this week.

A Facebook fundraiser for Raices started in June by a Californian couple had an initial goal of $1,500. It went viral, and within a couple of weeks generated more than $20m – three times the organisation’s annual budget.

The Raices office in San Antonio, Texas.

When splitting parents and children at the border became explicit White House policy in the spring – creating outrage, anguish and chaos – Raices was among the aid groups urgently seeking to locate and help families. In June a federal judge ordered families to be reunified, but about 3,000 children, mostly from Central America, were separated as they entered the US from Mexico.

The plan was criticised for inflicting fresh trauma on migrants escaping violence and hardship who had endured harsh journeys to reach the US, and for raising the prospect that some children will never be reunited with their parents.

“It’s been an awful run,” Nate Roter said. “It’s a horrible, horrible thing … But that’s why we’re here.”

A 32-year-old social worker with dark-rimmed glasses and flecks of grey in his beard, he “became very busy very quickly” after joining Raices in March. Like other staffers he worked exhaustingly long hours over the summer, sometimes seven days a week, grabbing a few minutes of light relief by watching the World Cup as he drank his morning coffee before heading to the office.

Inside the Raices office, clockwise from top: Santiago Campos, a staff attorney; Maria Guerra, left, the CFO, with Michelle Garcia, an accounting assistant; and Luz Varela, a legal assistant.

Raices is using part of its windfall to expand its bond programme, paying anywhere from $1,500 to more than $10,000 to free undocumented people from detention while their cases move through the legal system. It has covered about 240 bonds since March, 70 linked to the family separation crisis.

“It’s a short and direct way to get someone out of detention. Then the odds of them finding representation and winning their case increase dramatically,” Roter said.

This crisis was larger in scale and scope than anything we’ve ever seen Jonathan Ryan, Raices executive director

To pay bonds, staff visit a modern Immigration and Customs Enforcement (Ice) building in a San Antonio industrial estate. Opposite a Fresh from Texas food warehouse, it looks like a cross between a corporate headquarters and a castle. When the money is paid, migrants are released from facilities elsewhere. On a recent day, Raices covered the costs for two people picked up in a mass workplace raid near Paris, Texas, last month: one person separated under the zero-tolerance policy, and an asylum seeker in California.

“When people are out of detention they present as completely different people. Detention has a way of stressing them out and breaking them down,” Roter said. “Seeing that is really powerful but at the current moment there’s not much going on that’s too satisfying.”

Raices could call on between 1,000 and 2,000 volunteers before this year’s crisis; now there are more than 8,000, said Geovanie Ordóñez, the volunteer manager. “Folks would show up over the summer and say ‘how can we help?’ the 24-year-old said. Some meet released migrants at San Antonio’s bus station to offer advice. With the biennial Texas legislature meeting next January, Raices is now examining how to lobby for political change.

Nathan Roter: ‘It’s a horrible, horrible thing … But that’s why we’re here.’

Once a local operation run on a shoestring, today Raices has 10 offices across the state and about 160 staff who handle tens of thousands of cases each year without cost to clients. As a result of the influx of money it is hiring attorneys at a brisk pace, said Jonathan Ryan, the executive director since 2008. “This crisis was larger in scale and scope than anything we’ve ever seen,” he said.

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Raices is a logistical as well as a legal operation, aiming to foster connections between immigrants, family members and professionals who can help them access a range of social and medical services. It has been at the mall location since 2012.

Ryan’s office is so small that opening the door fully risks banging it against the desk. At first glance the framed letter on the wall looks like one of the academic certificates that hang nearby. In fact, it is a deportation order.

‘Needing help taught me to give help’

Born in Canada to Irish parents who moved to Mexico, Ryan is slight and softly spoken with blue-green eyes of piercing clarity. Now 41, he was a student in Austin in 2002 when he took an overnight bus to visit his family for Christmas. He fell asleep during the journey and when he woke up he realised that the bus had crossed the border but no one had stamped his passport.

When he tried to return to Texas after his visit he was stopped at a checkpoint and ordered into a van. As the door slammed and he was driven away to spend the night sleeping under a flea-bitten sheet in a frigid Mexican detention centre, Ryan begged a passerby for help.

Their interaction lasted only a few seconds: a man exclaiming, “what are you doing here?” and Ryan responding, “please help”. But he remembers the man’s compassionate concern and effort to connect by speaking English as a life-changing moment that set him on the path to becoming an immigration lawyer.

“This lesson to me on what it was to need help – particularly from a stranger – taught me what it was to give help,” he said. “My job moving forward was to attempt to be that guy; to fill that need.”

Eva Elisa, a volunteer from San Antonio’s Interfaith Welcome Coalition, greets newly released migrants.

When he started, the newly formed Department of Homeland Security was boosting its detection, detention and deportation machinery for a post-9/11 world in which immigrants are viewed as potential terrorists, with much of the apparatus based in south Texas.

“The Ice personnel of today is the personnel of the Obama administration on the whole, who are the personnel of the Bush administration on the whole,” Ryan said. What has ramped up since Trump’s election is a “systematic dehumanisation” of immigrants, he believes.

Still, though the work is even harder, the wave of attention and financial backing appears to have reinforced his sense of purpose. “It feels like there was a connection that happened this summer between us and a large segment of the American people,” Ryan said.

“What we learnt this summer is that those of us here in the trenches can have some hope knowing there are people out there in this country and around the world who still hold to the same ideals that we fight to protect,” he added. “It’s too bad that it takes such a horrific moment for us to connect.”