The US government and law enforcement agencies have hit out at developers and users of apps which track Immigration and Customs Enforcement (ICE), arguing they threaten the lives of agents.
The FBI says the man who targeted an ICE facility in Dallas - killing two detainees - had used these types of apps to track the movements of agents and their vehicles.
Special Agent Joseph Rothrock said: "It's no different than giving a hitman the location of their intended target" - a claim which has been disputed by the developer of one of the most popular apps.
BBC Verify has been looking at what the apps do and the potential impact they are having.
A number of apps have been released this year in response to President Trump's crackdown on illegal immigration and an upsurge in ICE raids.
The apps allow people to report the presence of ICE agents in their local areas which are then marked on a map to warn other users.
The most popular is ICEBlock, which was released in April and has been downloaded more than one million times.
It - and other ICE-tracking apps - can still be downloaded from the Apple Store, despite criticism of them from members of the Trump administration.
Joshua Aaron - who has worked in the tech industry for years - told BBC Verify why he developed ICEBlock.
"I certainly watched pretty closely during Trump's first administration and then I listened to the rhetoric during the campaign for the second. My brain started firing on what was going to happen and what I could do to keep people safe", he said.
In July, US Attorney General Pam Bondi accused Mr Aaron of "threatening the lives of our law enforcement officers throughout this country."
"We are looking at him, and he better watch out," she added.
Mr Aaron is undeterred.
"Anything that challenges what they're doing in this country and to this country, they're going to push back on... but they're not going to intimidate me. ICEBlock will be here for as long as it's necessary."
And he said specific criticism of him following the Dallas shooting was unjustified.
"You don't need to use an app to tell you where an ICE agent is when you're aiming at an ICE detention facility. Everybody knows that's where ICE agents are."
The BBC spoke to several undocumented migrants in Washington DC who use the apps to avoid ICE officers.
"It's scary. They could grab you anywhere," one of them said.
"I don't use it often but I think it can be useful. I've heard on the news that the government says it's dangerous [to ICE agents] but I've never heard of something really ever happening, at least not around here," he said.
Another said she found it hard to imagine a scenario in which an undocumented migrant would use the app to harm a law enforcement officer.
"We are here to work, and do not want problems. Nobody would want to make their situation here more difficult," she added.
