On July 1, Kathryn Steinle was shot and killed on a popular pier in San Francisco. Her murderer, 45-year-old Juan Francisco Lopez Sanchez, had taken sleeping pills he found in a dumpster hours before the shooting. He claims he found a gun wrapped in a T-shirt on a bench and accidentally let off a shot, not realizing he had struck Steinle until he was apprehended an hour later.

San Francisco has drawn criticism since the discovery that Sanchez was in the country illegally and had a long criminal history. He has been deported from the country five times – last in 2009, and is a seven-time felon. None of his felony convictions were for violent offenses and include illegal re-entry into the United States and drug-related convictions.

Sanchez, who was on probation in Texas at the time of the shooting, deliberately relocated to San Francisco while looking for work. Sanchez said he knew that San Francisco was a "sanctuary city," meaning it does not fully cooperate with authorities from Immigration and Customs Enforcement (ICE).

Mayor Edwin Lee says he is "concerned about the circumstances that led to the release of Mr. Sanchez," and called on agencies from all levels "to conduct quick, thorough and objective reviews of their own departmental policies and the decisions they made in this case."

On March 26, months before the shooting, Sanchez was transferred to San Francisco for a drug warrant and held in custody for a couple weeks. ICE asked San Francisco to notify the agency prior to his release on April 15 so that Sanchez could be apprehended and processed before he would be able to walk free.

However, the city's sheriff, Ross Mirkarimi, protested that ICE did not file "the proper legal instrument" necessary to hold Sanchez. As a sanctuary city, San Francisco does not automatically respond to ICE's detainer requests and instead requires an active warrant.

ICE officials claim that that the agency issues 200,000 detainers a year – a volume that would overwhelm the judicial system if it sought warrants in each case.

The tension between local governments and the federal government over immigration policy stretches back decades. San Francisco became a sanctuary city in 1989. Today, there are approximately 300 cities across the country that in some way limit their cooperation with ICE.

A major change in immigration enforcement took place in 2008, when the Department of Homeland Security (DHS) implemented the Secure Communities program. It called upon local authorities to fingerprint undocumented residents and alert ICE officials, whereupon ICE would send a "detainer request" asking the local authorities to hold on to the detainee for up to 48 hours before being picked up by ICE.

Critics charged that this policy was harmful, because it deterred undocumented residents from reporting crimes for fear of exposing their identities. Civil liberty advocates also pointed to the due process issues that arose from prolonged detentions, as well as the additional costs to local law enforcement. In 2012, a report found that Los Angeles County spent approximately $26 million each year complying with ICE's detention requests.

Starting in 2011, Illinois, Massachusetts, and New York announced they would not cooperate in the program, believing it to have been optional.

In 2013, California passed the Trust Act, which limited the situations in which the state would cooperate with ICE to those involving undocumented residents who had committed serious or violent crimes. The legislation was part of a national trend to prevent the detention of illegal immigrants who had committed minor offenses.

Supporters of the bill claimed that cooperation in Secure Communities led to the deportation of 100,000 residents of California, most of whom were not threats to public safety.

In the midst of the backlash from across the country, the federal government scrapped the Secure Communities in 2014. DHS Secretary Jeh Johnson announced in November that it would be replaced by the Priority Enforcement Program (PEP).

PEP differs from Secure Communities in two significant ways. First, it created a three-tier system to determine which illegal immigrants to pursue. The first priority would be those who posed a threat to national security and public safety; the second would be misdemeanants and new immigration violators; and the third would be all other violators.

Second, PEP no longer necessitates extended detentions. ICE officials can ask local authorities to notify the agency before an undocumented resident is released – as ICE did regarding Mr. Sanchez in San Francisco.

After the switch, some ICE officials expressed their disappointment with the lack of cooperation between local and federal authorities. In March 2015, ICE Director Sarah Saldaña expressed approval for legislation that would force law enforcement to comply with ICE's enforcement efforts.

When U.S. Rep. Mick Mulvaney (R-S.C.) asked, "Would it help you if we clarified the law to make it clear that it was mandatory that those local communities cooperate with you?" Saldaña replied, "Thank you, amen, yes."

The next day, however, she walked back her comments, declaring that mandatory compliance would be a "highly counterproductive step and lead to more resistance and less cooperation in our overall efforts to promote public safety."

In March, back in San Francisco, Sheriff Mirkarimi sent out a memo stressing the sheriff department's strict control over immigration enforcement after learning that some officials had been working with ICE without the department's approval.

After the death of Steinle, Mirkarimi stood by his city's immigration policies. He claimed that being a sanctuary city makes the city safer. He also said that he was justified in ignoring ICE's request to notify the agency prior to Sanchez's release.

Because of Sanchez's nonviolent history, his detention and transfer to ICE was not mandatory, according to California's Trust Act. His drug convictions made this detention optional and a matter of San Francisco's discretion.

ICE for not "upgrad its practices after all these years of seeing the imperfect deportation and transferring of people" and not issuing warrants that would enable cooperation between federal and local authorities.

Sheriff Mirkarimi blamed

"All ICE needs to do is to provide us that legal instrument," he has said. "A detainer is a cavalier way of saying that this is a legal instrument, but in truth, it is not.”