WASHINGTON (AP) — Immigration laws are federal laws -- not state laws -- and enforcement of immigration law is the domain of federal law enforcement. Homeland Security agents and officers are responsible for arresting migrants who are caught crossing the U.S. border illegally, whether from Mexico or Canada. They’re also responsible for arresting and deporting people who are in the country illegally.

That’s why the news in Texas over immigration enforcement is so unusual. Lawmakers there passed a bill that would make illegally crossing the border a state crime, which would theoretically allow state law enforcement to arrest migrants. But that clashes with how, generally, laws work in the U.S. The Texas law was supposed to go into effect this month, but there’s been a big back-and-forth in the courts about that.

Here’s a closer look:

FEDERAL V. STATE LAWS

In general, it works like this: State legislators make laws for their states that are enforced by state police or state patrol or other local law enforcement. The federal government does the same for the nation overall, and federal law enforcement agents like the FBI or U.S. Immigration and Customs Enforcement officers enforce those laws. Generally, federal laws take precedence over state laws.

READ MORE Court puts Texas law allowing police to arrest migrants who cross illegally back on hold Anticipation and anger on Texas border as strict immigration law again on hold Gov. Sanders deploys Arkansas National Guard to support southern border control efforts

But states can and often do pass legislation that encroaches on federal law. That’s when things get really murky.

THE TEXAS LAW

Texas Gov. Greg Abbott has been increasingly looking to take immigration matters into his own hands. The Republican governor is a huge critic of President Joe Biden and says the Democratic administration’s policies are failing.

In November, Texas passed a law known as S.B. 4 that would make it a state crime to cross into Texas from a foreign country anywhere other than a legal port of entry. It would be considered a misdemeanor on the first offense and a felony after that. The passage of this law would mean that state police officers could arrest any migrant caught crossing illegally. Previously, they were limited to arresting migrants found on private land for trespassing.

WHERE THINGS STAND

Right now, the law is on hold after lots of back-and-forth in the courts that has gone all the way to the U.S. Supreme Court. On Tuesday, the court allowed Texas to give local law enforcement the ability to arrest migrants. The court’s conservative majority rejected an emergency application from the Biden administration to stop the law from going into effect. The Biden administration argued the law is a clear violation of federal authority that would cause chaos in immigration law.

But then a federal appeals court issued an order that prevents Texas from enforcing the law. That’s where things are right now.

HOW DO IMMIGRATION ARRESTS WORK?

At the U.S. border, Border Patrol agents arrest people caught crossing illegally and send them to Border Patrol stations, where they are placed into deportation proceedings.

Some are then transferred to immigration detention, which is managed by Immigration and Customs Enforcement. Others are released into the U.S. to wait for their deportation hearings and appear for immigration court hearings. Immigration court is run by a third agency overseen by the Justice Department.

Customs officers, meanwhile, check identification at ports of entry, and they arrest anyone caught smuggling people over the border in vehicles.

ICE officers also arrest and deport people already in the interior of the United States. Usually these migrants are targeted because they’re accused of committing some other local crime. Other agents arrest employers suspected of mistreating migrants.

If someone is arrested by local or state police, it’s for a crime unrelated to immigration. They’re turned over to immigration authorities once they’ve been adjudicated.

HAS ANYONE BEEN ARRESTED?

Texas authorities had not announced any arrests made under the law while it was briefly enforceable.

As for federal arrests, yes. The Border Patrol, an agency under Homeland Security, arrests migrants caught crossing illegally. The patrol’s most recent data is from January, and it’s broken out by sector. In the Del Rio sector, it made 16,712 arrests. In the Rio Grande Valley, there were 7,340. Those arrests are down considerably from earlier months.

HAS ANY OTHER STATE TRIED THIS BEFORE?

Yes. Another border state, Arizona, passed a similar law in 2010 that authorized police to arrest migrants if there was probable cause they had committed an offense that would make them deportable, and it made it a state crime for “unauthorized immigrants” to fail to carry registration papers and other government identification. This case, too, went up to the U.S. Supreme Court.

But many of the provisions were struck down.

“The National Government has significant power to regulate immigration,” former Justice Anthony Kennedy wrote in the majority opinion. “Arizona may have understandable frustrations with the problems caused by illegal immigration while that process continues, but the state may not pursue policies that undermine federal law.”