A chilling alleged murder has thrown a major new wrinkle into an already complicated debate

When Juan Francisco Lopez-Sanchez stood in front of a San Francisco court earlier this year, he wasn't a possibly unstable, violent individual.

He was an undocumented immigrant facing decades-old drug charges who'd been convicted of several low-level drug crimes over the past several decades. He'd spent years in and out of federal prison.

Lopez-Sanchez languished in federal prison for more than a year after his sentence ended. In March, he was turned over to the San Francisco Sheriff's department by the Bureau of Prisons, after the agency discovered a 20-year-old warrant for marijuana charges in San Francisco, according to the American Civil Liberties Union.

The Los Angeles Times reported that prosecutors in San Francisco found the charges were too old to pursue, so Lopez-Sanchez was released onto the streets of San Francisco.

A few months later, he allegedly shot 32-year-old Kathryn Steinle, who was walking with her father in a popular tourist spot in San Francisco.

Lopez-Sanchez is an undocumented immigrant, and in the eyes of staunch immigration-enforcement advocates, Steinle's death is exactly the problem with the many left-leaning cities that have attempted to craft their own immigration-enforcement policies in the wake of congressional inaction.

San Francisco, along with other cities like New York and Los Angeles, is considered a "sanctuary city." It does not obey requests from ICE to indefinitely hold inmates beyond their detention date.

Along with refusing ICE detention requests, some sanctuary cities bar local police from asking for proof of citizenship and from arresting undocumented immigrants unless they are suspected of committing other criminal offenses. Some of these cities also allow undocumented immigrants picked up for low-level crimes to avoid deportation by serving jail time or paying fines.

But Steinle's murder has put sanctuary cities in a more controversial spotlight. Donald Trump, who has made illegal immigration a central theme of his nascent presidential campaign, has used Steinle's death as an example of why he says immigration laws need to be tightened. Fellow GOP candidate and US Sen. Rand Paul (R-Kentucky) introduced a bill last week that would bar sanctuary cities from receiving federal funding.

"Our nation now has whole cities and states who stand up and willingly defy federal immigration laws in order to protect illegal immigrants who have broken our nation's laws. This must end and it must end now," Paul said.

What are sanctuary cities?

Sanctuary policies emerged in the 1980s as a response to a massive wave of immigrants to the US fleeing civil wars in Latin America. Some churches and cities offered "sanctuary" to undocumented immigrants who feared deportation back to their war-torn homelands.

The sanctuary movement emerged as a response to the Ronald Reagan administration's decision to deport immigrants fleeing civil wars in which the administration itself was involved. As the Immigration Policy Institute has pointed out, the Department of Justice did not classify migrants fleeing from El Salvador and Guatemala as asylum seekers, as that would force the administration to acknowledge that atrocities were being committed by groups the White House backed.

But after Reagan left office, sanctuary policies found a new ally: police departments.

Civil-rights advocates argue that some of the biggest proponents for sanctuary policies are police departments, many of whom rely on communities for tips about crime.

A 2013 statement by the Major Cities Police Chiefs group declared that immigration enforcement was a "major concern" for departments working to build trust with communities. President Barack Obama's 21st Century Policing report also found that decoupling policing and immigration enforcement is key to improving community policing.

"At its core, [community policing] ensures that you have beat cops on the streets making relationships with the community, and that the community trusts that local police are there for their protection and that it's safe to have interactions with local police without risking the deportation of one's self or a family member," Melissa Keaney, a staff attorney at the immigration reform-friendly National Immigration Law Center, told Business Insider. "When local police become involved in immigration enforcement, the community loses trust."

Still, Steinle's death has sparked a wave of outrage on the right, who blame weak enforcement policies for her death.

"There's obviously no deterrence at this point with the lack of enforcement in the system as a whole," said Dan Stein, President of the Federation for American Immigration Reform.

'Political opportunism'

Republican presidential candidates have highlighted Steinle's killing, claiming that Democratic-instituted policies in cities like San Francisco are to blame for her death.

Trump, in particular, has railed against sanctuary cities. Last weekend, the real-estate mogul met with a group of Americans whose family members were killed by undocumented immigrants.

He proceeded to have a parent of one of the victims introduce him at two weekend speeches, during which he talked extensively about illegal immigration. He has repeatedly reinforced a claim on the campaign trail that the Mexican government is sending "rapists," drug runners, and other criminals to the US, and he has used Steinle's murder to bolster that argument.

"Where are the other candidates now that this tragic murder has taken place b/c of our unsafe border?" Trump tweeted.

Then on Monday, Paul introduced a bill aimed at blocking certain federal funding from sanctuary cities that don't comply with the detention requests from ICE.

Republicans appear to the momentum on the issue, as Democratic presidential candidates like former Secretary of State Hillary Clinton and former Maryland Gov. O'Malley (D), have backed sanctuary cities and highlighted immigration reform as key parts of their platforms.

That may be at odds with the American public. Though comprehensive immigration reform itself is relatively popular, the same is not true for sanctuary cities. According to a Rasmussen poll released earlier this month, 62% of Americans said they want the Justice Department to "take action" against sanctuary cities. Most voters also want to cut off federal funding to those cities, as well.

Some Democrats are already moving to distance themselves from sanctuary cities. As Politico reported, Sens. Dianne Feinstein (D-California) and Barbara Boxer (D-California) are considering their own legislation that would rein in sanctuary policies and help municipalities and states better comply with ICE detention requests.

But immigration-reform advocates say the current backlash against sanctuary cities misunderstands the benefits those policies have on communities.

The National Immigration Law Center's Keaney told Business Insider that Paul's legislation was short-sighted and potentially violated the 10th Amendment on the balance of power between states and the federal government by commandeering local jails to hold individuals for ICE. (Paul's office did not comment on the record for this story.)

"It's an example of political opportunism that's unhelpful and pretty disgusting given how tragic this whole series of events was," Keaney said.

"This undermines the core function of local police," she added, "to promote public safety and prioritize the investigation of serious and violent crimes."

Keaney pointed to specific parts of Paul's bill in an attempt to demonstrate how it prioritizes politics over practicality.

Right now, for example, the Bureau of Prisons processes requests from state and local agencies to hold or move detainees based on warrants for state criminal law violations. Paul's bill proposes prioritizing ICE transfer requests, which could possibly supersede criminal requests.

Keaney claimed that detainees held on criminal charges are "arguably a much great public safety concern than an administrative request from ICE on a civil-law violation," which Paul's bill proposes. More importantly, advocates for sanctuary cities policies point out that undocumented immigrants are not more likely to commit crimes than legal immigrants or naturalized citizens.

As The Washington Post has reported, undocumented immigrants are also less likely to be incarcerated than American citizens, and US citizens are more likely to be arrested for drug-smuggling along the US-Mexico border. A recent study by the American Immigration Council found that during immigration booms, crime rates actually drop.

'Moral turpitude'

Pro-border security groups say that simply because undocumented immigrants aren't more likely to commit violent crimes doesn't mean that people should forget that plenty still do.

"It's a small consolation to claim that we're not right in the center of the bell curve," said the Federation for American Immigration Reform's Stein.

"The truth of the matter is that people who are here illegally frequently have to commit crimes of what they used to call 'moral turpitude.' They commit fraud crimes, they don't file taxes, they use fraudulent ID, they use someone else's Social Security number," Stein added. "In the end, you're creating a culture of illegality."

Further complicating the picture are several recent federal court decisions that ruled that holding undocumented immigrants for ICE without probable cause is unconstitutional.

In the last several years, several courts have found that local agencies could be charged with violating the Fourth Amendment if they hold individuals on ICE requests because they have no probable cause to do so.

Both sides of the immigration debate agree that the federal government's failure to act has created confusion and chaos, and that Steinle's death raises questions about whether state, local, and federal law enforcement agencies can effectively prevent violent crime as different agencies butt heads over immigration enforcement.

"The system is completely dysfunctional, and we need a federal solution that doesn't involve the criminalization of an entire community," Keaney said.

Added Jonathan Blazer, an immigration expert at the American Civil Liberties Union: "The solution to the problem is not to try and force localities to engage in unconstitutional practices or make them choose between being hit with a lawsuit for liability for a constitutional violation and losing their federal policing funding."

On the right, proponents are pushing politicians hard to include tough border security measures and harsher punishment for repeat offenders. Fox News host Bill O'Reilly has collected 400,000 signatures and gotten an endorsement from Steinle's parents for a new proposal that would mandate a five-year prison sentence for undocumented immigrants who are deported but return to the United States.

But immigration and civil-rights advocates say that Americans should not rush to make reactionary policy in the wake of the alleged murder.

"They're not made in a vacuum based on a single incident that's bad policymaking," Keaney said. "We feel that the actions of one person should never be used to justify a policy that criminalizes a whole community."