San Francisco authorities released suspect Juan Francisco Lopez-Sanchez in April after dropping the drug charges on which they had asked federal authorities to turn him over -- even though federal officials had asked the city to let them know if they were going to cut him loose.
The city, however, doesn't honor such immigration detention requests under its 26-year-old sanctuary law.
San Francisco is one of dozens of cities, counties and states across the country that have laws, policies or regulations that prevent employees from cooperating with federal immigration enforcement efforts.
Here's some information about sanctuary cities and the debate surrounding them:
What is a sanctuary city?
There's no legal definition of a sanctuary city, county or state, and what it means varies from place to place. But jurisdictions that fall under that controversial term -- supporters oppose it -- generally have policies or laws that limit the extent to which law enforcement and other government employees will go to assist the federal government on immigration matters.
Some communities use nonbinding resolutions, executive orders, police department policies or orders, while others use laws to enforce such policies, according to the Congressional Research Service
In San Francisco, for instance, a 1989 law called the City and County of Refuge ordinance
prohibits city employees from helping federal immigration enforcement efforts unless compelled by court order or state law.
How many are there?
More than 200 state and local jurisdictions have policies that call for not honoring U.S. Immigration and Customs Enforcement detention requests, the agency's director, Sarah Saldana, told Congress in March.
What's their history?
The sanctuary movement is said to have grown out of efforts by churches in the 1980s to provide sanctuary to Central Americans fleeing violence at home amid reluctance by the federal government to grant them refugee status.
It's also a product of the long-running national immigration debate, in which officials in some more diverse and liberal communities sometimes take issue with aggressive immigration enforcement efforts.
What's the argument for sanctuary status?
Proponents say that by encouraging members of immigrant communities to work with police without fear of deportation, such policies help authorities improve public safety by helping authorities identify and arrest dangerous criminals who might otherwise go undetected.
"I firmly believe it makes us safer," San Francisco Sheriff Ross Mirkarimi told CNN's "The Lead with Jake Tapper." "We're a world-renowned city with a large immigrant population. ... From a law enforcement perspective, we want to build trust with that population."
Supporters say such policies are widely supported by police groups such as the International Association of Chiefs of Police and chiefs from the nation's largest police departments because they help communities unite to fight crime.
What do critics say?
"Unfortunately, a lot of cities in this country have decided they don't want to cooperate with ICE," Julie Myers Wood, former assistant secretary for Immigration and Customs Enforcement, told CNN on Monday. "They think that cooperating with ICE causes them problems with respect to the immigrant community and public safety, but in fact it does exactly the opposite, as we've seen here."
Such policies "ignore the fact that if the illegal aliens were removed from the U.S., they would not be here to become victims, and the predators would be out of the country, too," Ohio Jobs & Justice PAC, which opposes sanctuary policies, says on its website.
"Sanctuary policies -- official or otherwise, result in safe havens (or safer havens) for illegal aliens involved in a variety of criminal enterprises -- since their illegal schemes are less likely to be uncovered and face less risk of deportation if caught by local law enforcement," the website says.
"Sanctuary policies also provide an environment helpful to Latin American drug cartels, gangs, and terrorist cells -- since their activities are less likely to be detected and reported by law enforcement."
Some Republican presidential candidates have used similar language. Donald Trump has blamed immigration policy for Kate Steinle
's death. Another Republican, Jeb Bush, agreed, saying such policies encourage such crimes.
What has the federal government said?
In March, Saldana, the ICE director, drew heat from immigrant rights supporters after appearing at a congressional hearing to endorse efforts to rein in the sanctuary movement.
In written testimony submitted to the House Oversight and Government Reform Committee, Saldana said that a significant factor affecting efforts to deport undocumented immigrants "has been the increase in state and local jurisdictions that are limiting their partnership, or wholly refusing to cooperate, with ICE immigration enforcement efforts."
"While the reasons for this may vary, including state and local legislative restrictions and judicial findings of state and local liability, in certain circumstances we believe less cooperation may increase the risk that dangerous criminals are returned to the streets, putting the public and our officers at greater risk," she said.
During questioning, she was asked if she would support a new federal law mandating local cooperation.
"Thank you, amen," Saldana reportedly answered, according to media reports.
That stance quickly drew the attention of critics such the American Civil Liberties Union, which said in a blog post that Saldana's comments "insulted all the states and localities across the country who have wisely decided to stay out of immigration enforcement."
She quickly issued a statement saying that any such legislation would "be a highly counterproductive step and lead to more resistance and less cooperation in our overall efforts to promote public safety."
It's sure to become a point of discussion on the presidential campaign trail, in local elections and among the professional debating corps, but given the complexities of the issue and decades of difficulty reaching any consensus on the issue, it's far less certain the incident will result in any widespread changes.