In June, a woman named Kathryn Steinle was murdered in San Francisco — and the man charged in her death, Francisco Sanchez, was an unauthorized immigrant and five-time deportee. For conservative immigration hawks, the Steinle murder fulfilled a prediction they'd been making for years: that the "sanctuary city" policies San Francisco and other cities use to limit cooperation with federal immigration agents were a menace to public safety.

Fast-forward to today, and the House has already passed an anti-sanctuary city law. The Senate, for its part, is considering a bill called "Kate's Law," as well as other proposals.

But it's not clear that San Francisco deserves blame for Sanchez going free — or if these bills will actually solve the problem they want to solve. Meanwhile, forcing cities to play a larger role in enforcing federal immigration laws could endanger public safety by making unauthorized immigrants reluctant to talk to the police.

What is a sanctuary city?

Politicians use the phrase "sanctuary city" to mean plenty of different things — including to imply that a city doesn't let the federal government deport any of its residents. But cities can't stop the feds from enforcing immigration laws. Rather, the term refers to cities that limit when they'll help federal immigration agents detain unauthorized immigrants.

The issue comes up when the federal government asks a local jail to hold an unauthorized immigrant after he'd normally be released, so that the feds have a chance to pick him up. Some cities (and states) have policies in place saying they'll only hold the immigrant for extra time if he's committed a serious crime. Otherwise, they'll release him on the same schedule as any other detainee.

What's confusing about this is that when immigration hawks started complaining about "sanctuary cities" 10 years ago, they were referring to something else: cities that didn't inform the federal government when they had custody of an unauthorized immigrant. Under the Obama administration, that's not really possible anymore: The fingerprints of anyone booked into a jail get sent through an immigration database. But it's still the definition of "sanctuary city" that a lot of people operate under.

Why don't these cities want to work with the feds on immigration?

The job of local cops isn't to enforce every local, state, and federal law. It's to enforce local laws and protect the safety of their residents. And cities worry — with good reason — that turning over unauthorized immigrants more easily would get in the way of promoting public safety.

Most unauthorized immigrants aren't criminals. But they could be victims, or witnesses, of crimes. And local police want to be sure that unauthorized immigrants who are victims of crime aren't too afraid of the police to file a report. The city can't do much about a generalized fear of law enforcement because of immigration status, but they can make sure their actions don't encourage people to think of them as an arm of federal immigration agents.

So for years, cities and states have been fighting the Obama administration over when they have to cooperate. (For more about the conflict between cities and the federal government on this, see my article from last year.) After years of tension, the administration has tried to reboot its local-federal coordination program to address cities' concerns; as part of the president's executive actions on immigration last fall, they instituted a new policy that lets cities negotiate up front when to detain unauthorized immigrants. But they're still struggling to win back cities' trust. And now that dynamic has been totally overturned by the Steinle murder and ensuing congressional scramble.

Why is this a big deal now? Is it because of Donald Trump?

Well, not exactly. Trump has certainly been telling anyone who will listen that immigrants are rapists and murderers. What started as an offhand comment during his campaign launch has become the theme of his presidential campaign, with a massive rally in Phoenix on the theme and a trip to the US/Mexico border Thursday. It's hard to say that all this had no effect on the current sanctuary city controversy. And Democrats are more than happy to call the anti–sanctuary city bill passed by the House the "Donald Trump Act," because Trump is saying crazy things that are embarrassing many Republicans.

But while Trump claims to have single-handedly brought the issue of unauthorized immigration into the public eye, he's not really the genesis of the current anti-sanctuary push. That would be the murder of Kathryn Steinle in San Francisco in June, for which an unauthorized immigrant and five-time deportee called Francisco Sanchez has been arrested.

Steinle's murder immediately became a big story in San Francisco, and national conservative media picked up on it — and used it as a way to attack San Francisco's "sanctuary city" policy of not always honoring federal hold requests. Steinle's family has helped this narrative, since they've been outspoken about wanting more laws preventing people like Sanchez from being out on the streets.

These conservatives have seized on the fact that in March, Sanchez was sent from federal prison (where he'd finished a prison sentence for illegally reentering the US after his last deportation) to a San Francisco jail because he had an outstanding warrant from 1995 for marijuana possession there. But prosecutors decided not to charge him on the 20-year-old pot charge — and San Francisco's immigration-hold policy is that drug offenders don't get held for extra time so the feds can pick them up. So Sanchez was released, while in another city with a more federal-friendly policy he would have been held for immigration agents to pick him up.

But San Francisco's policy isn't the only reason — or even the main one — that Sanchez was in the US and out of custody when Steinle was killed. It's still not clear why Sanchez was sent to San Francisco after his prison term to begin with — after all, he could have been sent to immigration agents straight from federal prison. And if federal agents were really particularly worried about Sanchez going free, they could have apprehended him in the months between his release and Steinle's murder.

So opponents of tougher immigration enforcement are stressing that you don't need to change policies like San Francisco's — and erase any progress they've made on community trust — to prevent tragedies in the future. But for immigration hawks, the Steinle murder confirms something they've long suspected about sanctuary cities: that they let criminals go free and commit new crimes.

So what is Congress actually trying to do to get rid of "sanctuary cities"?

Republicans have asked Department of Homeland Security Secretary Jeh Johnson why the federal government can't simply require all local law enforcement to hold immigrants for extra time whenever asked. Unfortunately for them, there have been several court cases challenging this policy — and many courts have ruled that it's unconstitutional to simply hold immigrants in jail while waiting for federal agents to swing by.

So while there are a bunch of bills floating around Congress in response to Steinle's murder, they typically fall into two main categories: banning cities that limit immigration enforcement from getting certain federal grants, and extending prison sentences for illegal reentry. The bill just passed by the House would do the former; the Senate has introduced several bills to do the latter.

The House bill would prevent police departments that restrict "the exchange of information" about unauthorized immigrants from being able to get major federal policing grants, including the Byrne JAG and COPS grants. The thinking is that no local government would be willing to forgo that much money, so they'd change their policies about immigration holds to get back on the federal government's good side.

Here's the problem, though: It's not at all clear what restricting "information" means under current policy. Ten years ago, when cities were deciding when to notify the federal government, that definition of sanctuary city made sense; right now it doesn't. If a city refuses to hold an unauthorized immigrant for extra time after, say, charges are dropped against him, but notifies the federal government that it's releasing the immigrant — which is what it's supposed to do under the new federal policy — is the city restricting the exchange of information? Right now it looks like that would be up to the federal government to decide.

And while the Obama administration doesn't like cities deciding when to cooperate on immigration, it's certainly surprising that Republicans would put enforcement power in an administration they routinely accuse of failing to enforce immigration laws.

What about the bill called "Kate's Law"?

The second kind of bill — providing a mandatory minimum sentence of five years to anyone convicted of illegally reentering the US after deportation — doesn't technically have anything to do with sanctuary cities, but it's being called "Kate's Law" after Steinle, so it's part of the same conversation. The logic behind the bill is that since Sanchez had entered the US so many times after getting deported, the US must not be doing enough to deter people from trying to come back — so they need to get longer prison sentences for it.

Longer prison sentences don't deter crime. And the bill would substantially increase the prison population: More than a quarter of all cases in federal court last year were for illegal reentry, and the average sentence right now is only about 18 months. The money the federal government could hypothetically save by deporting a repeat offender fewer times would be more than wiped out by the increased costs of keeping immigrants in custody for longer. But it's not a tremendous change to immigration policy. Illegal reentry is already a federal crime — and it's already something that immigrants crossing into the US are charged with all the time.

As standalone bills, these are likely to be vetoed by Obama. The administration has already released a statement saying it would "recommend" a veto on the bill the House passed. The question is whether they'll be incorporated into the final versions of funding bills — like the Homeland Security appropriations bill. If that happens, the administration will have to choose between siding with congressional Republicans and siding with the cities that have caused it such a headache over the last several years.