Immigrant advocates and sympathizers are warning about arrests around the country as early as Sunday.

The anticipated sweep is expected to be similar to operations that authorities have regularly done since 2003. They often produce hundreds of arrests.

This one is different because President Donald Trump announced Monday on Twitter that it would be the start of an effort to deport millions of people in the country illegally, a near-impossibility given limited resources of U.S. Immigration and Customs Enforcement, which makes the arrests and carries out deportation orders.

It’s also slightly unusual to target families — as opposed to immigrants with criminal histories — but not unprecedented. The Obama and Trump administrations have targeted families in previous operations.

Here are some questions and answers about how ICE operates:

WHAT THEY CAN DO

Immigration and Customs Enforcement is in charge of arresting and deporting immigrants who lack legal status.

One common method of finding and arresting people who are known to be in the country illegally is agreements between ICE and local jails around the country to hold people arrested on crimes past their release date so that ICE can look into their status. These are known as “detainers,” but they’ve become increasingly unpopular among local governments, many who say they risk legal action and that they shouldn’t be doing the work of federal authorities.

The agency also arrests people the old-fashioned way, by tracking people down and showing up at their homes or workplaces.

But the amount of resources and staff limit 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 over 158,500 immigrants in the country illegally, an 11% increase over the prior year and the highest number since 2014. The agency says 66% of those arrested are convicted criminals.

Last month, ICE officers arrested 900 people during a three-week sting in California.

The agency announced last week that it arrested 140 people, including 45 in Illinois, during a sting in the Midwest that lasted five days.

Although ICE arrests people a variety of ways, it’s the larger enforcement operations such as a workplace sting that draw the most attention.

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.

“I think what people forget is these operations go on on a regular basis,” said Art Acevedo, the police chief in Houston, one of the cities believed to be targeted in an upcoming sweep.

WHAT IT LOOKS LIKE

Authorities typically have a list of people they are targeting in any operation. They visit a targeted person’s known addresses, usually a home or workplace, and seek to detain that person. They may ask family members, neighbors, co-workers, or managers about the whereabouts of the person they want to arrest.

Authorities typically obtain an administrative warrant giving them permission to detain a person for violating immigration law.

ICE agents can arrest people they discover to be in the U.S. illegally while searching for people on their target list. People who answer ICE agents’ questions about someone else sometimes end up arrested themselves. In one case in Houston last year, a young father of five was arrested in the parking lot of his apartment building after ICE agents asked him about people who lived nearby, then demanded his identification and eventually detained him.

These “collateral” arrests can comprise a large portion of the arrests in any operation. In one December 2017 operation in northern Kentucky, just five of the 22 arrests ICE made were of people it originally targeted, according to agency documents released under the Freedom of Information Act.

WHAT WE’RE EXPECTING NEXT

The Washington Post and Miami Herald reported that 10 cities are expected to be targeted in raids starting Sunday. The Herald reported those cities are Atlanta, Baltimore, Chicago, Denver, Houston, Los Angeles, Miami, New Orleans, New York, and San Francisco.

ICE officials said this week that they had sent about 2,000 letters in February to people in “family units” who had already received final orders to leave the country. The people who received those letters may be the targets of the enforcement operation.

Acevedo, the Houston chief, said ICE officials this week declined to provide him with any information about the expected weekend operation besides saying they had ongoing enforcement operations. He criticized President Donald Trump’s tweets Monday saying that agents would begin removing “millions of illegal aliens.”

“It instills fear,” Acevedo said. “We rely on the cooperation of that population to keep all Americans safe, all residents safe, and all members of society safe. … When you say you’re going to go arrest millions of people, that has a chilling effect on the cooperation.”