How do immigration authorities make arrests?

After postponing an immigration-enforcement operation late last month, the Trump administration plans to go ahead with the raids as soon as this weekend. The sweep is expected to be similar to others that authorities have done regularly since 2003 and often netted hundreds of arrests.

This one is different because President Donald Trump tweeted in June that it would be the start of an effort to deport millions of people who are in the country illegally. That’s a near-impossibility given the limited resources of Immigration and Customs Enforcement. It’s also slightly unusual to target families — as opposed to immigrants with criminal histories — but not unprecedented.

Here are some questions and answers about how ICE operates:

WHAT CAN THEY DO?

The agency is in charge of arresting and deporting immigrants who lack legal status.

One common method of finding people who are known to be in the country illegally is for local jails to hold those who have been arrested on crimes past their release date. That allows ICE to look into their status. These are known as “detainers,” but they have become increasingly unpopular. Some local governments complain that detainers put their officials at legal risk and that local authorities should not be doing the work of federal authorities.

ICE also arrests people the old-fashioned way, by tracking them down and showing up at their homes or workplaces. But limited staff and resources constrain their ability to make multiple large-scale arrests at a time.

WHAT HAVE THEY DONE?

Last fiscal year, ICE’s Enforcement and Removal Operations unit arrested more than 158,500 immigrants in the country illegally, an 11 percent increase over the prior year and the highest number since 2014. The agency says 66 percent of those arrested were convicted criminals.

In Texas, ICE’S Homeland Security Investigations unit, which enforces immigration laws at workplaces, arrested 280 employees at a company in Allen, Texas, in April, saying it was their biggest worksite operation in a decade.

HOW DOES A SWEEP HAPPEN?

Authorities typically have a list of people they are targeting and administrative warrants giving them permission to detain those people for violating immigration law. They visit a targeted person’s addresses, usually a home or workplace. They may interview relatives, neighbors, co-workers or managers.

ICE agents can also arrest people they discover to be in the U.S. illegally while searching for targets on their list. People who answer agents’ questions about someone else sometimes end up arrested themselves.