PHOENIX — As a helicopter circled overhead, 15 trucks and about 30 agents, some armed with long rifles, swarmed into a medical aid camp in the Arizona desert last week to capture four immigrants.
The use of military tactics and a search warrant suggested a new strategy by the Border Patrol leadership in the region. The agency had previously abided by an informal, Obama-era agreement allowing migrants to seek medical help at the camp without fear of arrest.
Thursday’s operation was part of a US government crackdown along the border with Mexico, but volunteers with the aid group No More Deaths/No Mas Muertes described it as a ‘‘staged military siege’’ that they said could put lives at risk.
The group provides refuge and water for migrants, said Catherine Gaffney, a longtime volunteer who was present during the arrests. ‘‘The type of operation they are doing, for me, is unprecedented,’’ she said.
Gaffney said a camp doctor asked the agency late Thursday morning for more time to treat the men, who had suffered from heat-related illnesses and needed an additional 24 hours of supervised care.
The Border Patrol said talks between agents and camp representatives on gaining access to question the men about their citizenship and legal status failed. It said it had no recourse but to obtain a search warrant.
Authorities said they had been monitoring the men since Tuesday, when they walked into the camp after spending several days in the desert. Temperatures were in the triple digits on the day of the raid.
Customs and Border Protection, which runs the Border Patrol, said one of the men arrested was identified as Lucindo Díaz-Hernández, who had a prior felony conviction for drug possession and had spent five years in prison for drug trafficking. He also had been previously deported, it said.
Alicia Dinsmore, a No More Deaths spokeswoman, said the camp is staffed by trained volunteers who have first-responder certification and provide care, food, and water.
The group has been providing aid for 13 years and has a verbal agreement with the Border Patrol’s Tucson Sector to operate there. ‘‘This incident was a targeted attack on humanitarian aid,’’ Dinsmore said.
The enforcement action comes as President Trump has made securing the border a top priority of his administration, including a campaign promise to build a wall along the border with Mexico.
His presidency has coincided with a big drop off in immigrants crossing the border from Mexico, but immigration authorities have been arresting more people who are in the country illegally, and doing so in places they had previously avoided, like courthouses.
Gaffney said volunteers were escorted to a different part of the camp as a helicopter circled overhead and the agents arrested the four men, who are between the ages of 19 and 40.
‘‘They didn’t need 30 agents to apprehend four sick people,’’ Gaffney said, adding that the agency’s public relations team filmed the encounter.
The Border Patrol says agents found the men were in good health but took them to a hospital as a precautionary measure.
The agency’s Tucson Sector, which covers most of Arizona, says it has launched a campaign to warn migrants about the dangers of crossing the border in the summer. The sector has 34 strategically placed rescue beacons that migrants can activate for rescue. More than 200 agents in the Tucson Sector are emergency medical technicians, and about 25 are paramedics, the agency said.
Agents in that area conducted more than 1,400 rescues and reported 84 deaths last fiscal year, according to agency statistics. So far this year through April, agents in the Tucson Sector have rescued 160 people and reported 14 deaths.
‘‘Our primary mission is to conduct law enforcement operations along the border and in the course of our duties we’re often the first responders to emergency situations,’’ Border Patrol spokesman Vicente Paco said. ‘‘We are one [of] the largest agencies that have resources in the desert where we respond.’’
Paco said the agency understands the mission of No More Deaths but doesn’t condone its actions because it encourages illegal immigration.
‘‘We do believe in saving lives, and we have multiple resources such as the rescue beacon towers,’’ he said. ‘‘Regardless of their immigration status, we render aid.’’
Gaffney said the camp would carry on with normal operations.
‘‘We’re not able to stop our work, and we’re not going to let the government create a trap for people seeking help. I think that’s what Border Patrol’s intention is to deter people from seeking help when they need it and to entrap anybody who does,’’ she said.