BRACKETTVILLE, Texas—As this South Texas border county has seen a jump in illegal border crossings, Sheriff Brad Coe is cooperating with groups of armed private citizens to help patrol the border and arrest migrants for trespassing.

The Kinney County sheriff has been in regular contact for months with a group of men donning body armor and rifles while patrolling to look for migrants. Another armed group offered use of a high-tech drone, and went on a patrol along with the sheriff. It has also pursued potential partnerships...

BRACKETTVILLE, Texas—As this South Texas border county has seen a jump in illegal border crossings, Sheriff Brad Coe is cooperating with groups of armed private citizens to help patrol the border and arrest migrants for trespassing.

The Kinney County sheriff has been in regular contact for months with a group of men donning body armor and rifles while patrolling to look for migrants. Another armed group offered use of a high-tech drone, and went on a patrol along with the sheriff. It has also pursued potential partnerships with private security firms.

A new Texas law allows migrants to be arrested for trespassing and has attracted such groups, and local officials are either offering tacit support or openly welcoming them. The groups say they aim to stop the migrants they find and pass them off to law enforcement to arrest.

Mr. Coe, a former Border Patrol agent, said he is looking for any help he can get in trying to drive migrant traffic elsewhere.

“The whole premise is if [migrants] know they’ll be arrested, they’ll go somewhere else,” he said.

State authorities have resisted these private operations, saying they are concerned about armed civilians trying to act as law enforcement. On Wednesday, the ACLU and nine other organizations filed a complaint to the U.S. Department of Justice, requesting that it investigate Texas state agencies and local governments involved in the effort to arrest immigrants. Among their concerns, they said, is the support, tacit or open, Kinney County has given such private groups.

Patriots for America members, with leader Samuel Hall on the far right, patrolling for immigrants near the border recently.

Photo: Samuel Hall

It isn’t clear how many migrants the private groups have stopped or arrested.

Over the summer, county leaders and Gov. Greg Abbott declared the border situation a disaster, a classification typically used for events such as hurricanes. Mr. Abbott, a Republican, began an experimental state effort to enforce federal immigration law by using state troopers to arrest and jail migrants on misdemeanor trespassing charges.

Of some 2,000 arrests by state troopers under the program, named Operation Lone Star, the majority have occurred in Kinney County. The volume has overwhelmed courts in the small community, which has had just one jury trial in the past seven years, leaving migrants in jail for months before seeing a judge. The arrests have resulted in few convictions.

Kinney County, a ranching county of 3,569 people along 13 miles of Rio Grande river frontage, sits midway along Texas’s 1,254-mile border with Mexico and was once one of its quietest regions for illegal border crossings. Shifting immigration patterns, combined with an upswing in illegal crossings in the wake of the Covid-19 pandemic, changed that.

Samuel Hall, leader of a North Texas based group called Patriots for America Militia, said his group of volunteers conducts patrols on public property, looking for migrants who may have crossed the border illegally, administering humanitarian aid when necessary and calling authorities when needed.

Kinney County Sheriff Brad Coe said he is looking for any help he can get in trying to drive migrant traffic elsewhere.

Photo: Elizabeth Findell/The Wall Street Journal

Mr. Hall said he doesn’t forcibly detain anyone, which would be illegal. That said, he said he realizes that foreigners encountering heavily armed people may not understand that.

Mr. Coe meets with Mr. Hall periodically, Mr. Coe said, but hasn’t worked with him officially. “I don’t know if they’ve caught anything or not,” Mr. Coe said of the group’s efforts to intercept migrants. Mr. Hall said he has worked closely with Mr. Coe and has called his office whenever the group has encountered immigrants.

When Patriots for America arrived in October, the sheriff welcomed the group and considered deputizing some of its members to his department, he said. But the plan received pushback from the Texas Department of Public Safety, he said. That made some county officials worry that the state would pull resources from the county if it partnered with private militias, some officials said.

Travis Considine, a spokesman for Texas Department of Public Safety, said the agency doesn’t support partnering with militia-type groups. He said the agency had never threatened to pull resources from the county. “We do not want to work with [militias], but we do not control the sheriffs,” he said.

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This year, the county began seeing high-speed chases and reports of migrants breaking into homes looking for food and water, Mr. Coe said. Ranchers have been rattled by strangers walking across their property and frustrated by damage to fences that caused livestock to escape. At least 16 migrants have died in the brush there since April, Mr. Coe said. No violent crimes have been reported.

Kinney County has embraced the state’s trespassing arrest efforts. It is a deeply conservative area where the sheriff has plastered Donald Trump stickers on his work desk and keeps a life-size cutout of Vice President Kamala Harris in his office to mock during televised news interviews.

Mr. Coe has pressed trespassing charges on behalf of some property owners, saying he is acting as the agent for several ranches. Driving west toward the border through ranch land, he rattles off the number of arrests of migrants and smugglers the Department of Public Safety has made there in recent days.

The county has sought to bring in additional private funding for border efforts. The home page of its governmental website declares the county is “drawing a line in the sand” and links to a site soliciting donations to respond to the border “invasion” that has raised nearly $21,000.

Samuel Hall, right, addressing Kinney County commissioners during a meeting this week.

Photo: Elizabeth Findell/The Wall Street Journal

At a county commissioners meeting this week, Patriots for America’s Mr. Hall thanked them for their support.

After the meeting, County Judge Tully Shahan, the local elected executive, told Mr. Hall, “Don’t come here and use us as a photo op.” Mr. Shahan, outside the meeting, said Mr. Hall’s group was uninvited in county efforts and said he supports Texas DPS. He declined to comment further.

Another commissioner, Mark Frerich, stopped Mr. Hall outside the meeting to express a different view, that he supports having the group there. “Don’t take it to heart, you’re doing good,” Mr. Frerich said.

The county has sought out other private, armed groups to do immigrant patrols. In July, officials approved a letter of intent to hire operatives from Garrison Trading Associates, a private security contractor which cited work in Iraq and Afghanistan. The deal fell through, in part because of a leadership shift at Garrison and in part because the county couldn’t afford it, those involved said.

In September, Mr. Coe welcomed Christie Hutcherson, head of Women Fighting for America, a group that has patrolled in various border areas and sought to partner with law enforcement. The sheriff partnered with Ms. Hutcherson and members of her organization who were armed to patrol the county at night, where a drone helped locate a large group of migrants, he said. The group has submitted a proposal to formally partner with the county.

Mr. Coe said the county couldn’t afford Ms. Hutcherson’s proposal, which he described as a cost higher than the Sheriff Department’s annual operating budget. Instead, he this week asked commissioners for permission to fund a similar drone from the online donations the county has collected.

The Southern Poverty Law Center, which tracks extremist groups, flagged Ms. Hutcherson’s group for spreading extreme conspiracy theories.

Ms. Hutcherson said that her organization isn’t a militia. It believes in working through law enforcement channels, she said. She had not seen how the SPLC characterized her, and said “I’m not an extremist. I’m a patriot.”

A North Texas man, who a spokesman said was in Kinney County patrolling for migrants on a private ranch, was arrested there this week on charges of participating in the Jan. 6 attack on the U.S. Capitol. Federal authorities said he was a member of the Patriot Boys Militia.

Kate Huddleston, a staff attorney with the ACLU, said the organization is concerned about both the actions of private organizations and the movement of the county toward potentially partnering with one of them. A group interfering with anyone’s free travel based on a suspicion that they might be migrants is an interference in others’ rights, Ms. Huddleston said.

“Kinney County has not just sought to partner with militias, but also war contractors,” she said. “That indicates a level of militarization that is particularly concerning and alarming in the potential for violence.”

Mr. Hall said objections won’t dissuade him.

“We’re here because God called us and we’re going to keep operating,” he said.

Members of Patriots for America Militia outside the Kinney County Courthouse.

Photo: Elizabeth Findell/The Wall Street Journal

Write to Elizabeth Findell at Elizabeth.Findell@wsj.com