If you know someone who is being held in immigration detention but you don't know where, you can search for them using an online tool. The US Immigration and Customs Enforcement division has an Online Detainee Locator System (ODLS) that you can use to find someone with only a minimal amount of information.
You can find someone by their A-number (short for alien registration number) and country of birth. This, according to ICE, is the best way to search. But even if you do not know the A-number, you can input a name and birth country and use the tool to locate an inmate in immigration detention. Let's consider what information the tool will provide, and what you will not find.