Here's a look at some of the statistics, where they come from and what people on different sides of the debate say about them.
That's the latest estimate of the number of unauthorized immigrants in the United States, according to the Pew Research Center
. And it's less than 4% of the total U.S. population
The number peaked in 2007, according to Pew, when there were an estimated 12.2 million unauthorized immigrants in the country. But since 2009, it's "remained essentially unchanged," Pew reports
, as the numbers of undocumented immigrants entering and leaving the United States "have come into rough balance."
The number of undocumented immigrants deported last year who were convicted criminals, according to Immigration and Customs Enforcement
That's 56% of last year's total number of deportations, according to ICE, and it's a group that the agency says it's putting first when it comes to deciding which cases to prioritize.
Ask someone like Republican presidential contender Donald Trump about this number and you'll get an earful about how this statistic and others show rampant crime among undocumented immigrants.
Immigrant advocates say that's far from the truth, adding that it's important to look at how the term ICE uses when it talks about convicted criminals -- "criminal alien
" -- is defined.
"Government statistics on who is being removed from the country can be somewhat deceptive," says Walter Ewing, a senior researcher for the American Immigration Council who helped author a report released this week that argues immigrants are less likely to be criminals than native-born U.S. citizens
"Immigrants who experience even the slightest brush with the criminal justice system, such as being convicted of a misdemeanor, can find themselves subject to detention for an undetermined period, after which they are expelled from the country and barred from returning," the report says. "In other words, for years the government has been redefining what it means to be a 'criminal alien,' using increasingly stringent definitions and standards of 'criminality' that do not apply to U.S. citizens."
The number of people released from immigration custody who were later charged with murder between 2010 and 2014, according to figures from the Department of Homeland Security cited in a recent letter from two U.S. senators
. That's about a thousandth of a percent of the total estimated number of unauthorized immigrants in the United States.
Sen. Jeff Sessions and Sen. Chuck Grassley, both Republicans, have been pointing to this number as they push federal authorities for answers, arguing that "countless innocent Americans every year are the victims of crime perpetrated by deportable criminals."
And the recent San Francisco shooting has added fresh fuel to the debate over whether authorities are doing enough to keep harmful offenders off the streets -- and kick them out of the United States.
But other officials have stressed that it's important to look at the big picture and not to make knee-jerk decisions based on individual cases.
"On the issue of immigration," California Attorney General Kamala Harris said this week, "our policy should not be informed by our collective outrage about one man's conduct."
The number of inmates in state and federal prisons who are not U.S. citizens, according to the latest prison population report from the Bureau of Judicial Statistics
. That's about 5% of the total prison population.
In his recent push for building a massive wall on the southern U.S. border, Trump has said that there are "hundreds of thousands" of undocumented immigrants in the nation's state and federal prisons. It was a claim that the PolitiFact fact-checking website gave a "mostly false" rating
"The basic claim is at best unsustainable or more likely pure fiction. A fact created out of thin air," Ramiro Martinez, a criminal justice professor at Northeastern University, told PolitiFact.
The bottom line, PolitiFact says, is that it's unclear just how many undocumented immigrants are currently in prison, because the available statistics don't provide details about inmates' immigration status.
Analysts on both sides of the immigration debate do agree on one thing: There's a lack of good data about this. But as to how likely it is for immigrants to commit crimes, that depends on who you ask.
In a 2009 report
arguing that immigrants have "high rates of criminality," the Center for Immigration Studies pointed to statistics that non-U.S. citizens represent a quarter of the U.S. prison population.
This week's report from the American Immigration Council counters that immigrants are less likely to commit crimes and are overrepresented in the federal prison system. Citing the American Community Survey, the report also notes that the percentage of foreign-born men in the United States who are incarcerated (1.6%) is less than the percentage of U.S.-born men who are imprisoned (3.3%). And the reason they're behind bars is often tied to immigration offenses.
"While some may be there for committing a serious criminal offense, a great many more may be there because of an immigration violation," the report says.
The number of so-called detainer requests issued by Immigration and Customs and Enforcement and sent to local authorities from 2008 to 2012, according to the Transactional Records Access Clearinghouse at Syracuse University
More than three-quarters of those were for immigrants who had no criminal convictions on their records. For the ones who'd been convicted, only 8.6% were charged with serious offenses, based on federal standards.
Federal immigration authorities use detainers to ask local agencies to hold unauthorized immigrants and eventually hand them over to the Department of Homeland Security.
It's a controversial approach that's long drawn fire from immigrant rights advocates, who argue that the feds have used the tool inappropriately to detain and deport people who don't deserve it.
It's a situation that's been in the spotlight with this month's shooting in San Francisco. Critics of the city argue that officials there erred when they didn't let Homeland Security know they were releasing suspect Juan Francisco Lopez-Sanchez.
In an interview with CNN on Tuesday
, Democratic presidential hopeful Hillary Clinton said San Francisco should have listened to the Department of Homeland Security and made a mistake when it didn't send Lopez-Sanchez packing.
"I have absolutely no support for a city that ignores the strong evidence that should be acted on. ... If it were a first-time traffic citation, if it were something minor, a misdemeanor, that's entirely different," she said. "This man had already been deported five times. And he should have been deported at the request of the federal government."
San Francisco Sheriff Ross Mirkarimi said his city has a good reason for their approach.
"Because of what has not been reconciled on the federal level," he says, "local governments and state governments are devising new laws."
The number of ICE detainers state and local enforcement authorities declined to honor last year.
"This required ICE to expend additional resources attempting to locate, apprehend and remove criminal aliens who were released into the community, rather than transferred directly into custody," ICE said in its report on last year's deportation statistics
But when it comes to immigration detainers, there's a long history of distrust between local and federal agencies, says Ruben Rumbaut, a professor of sociology at the University of California at Irvine.
"The majority of people that were put in ICE detainers were in the end deported out of the country for reasons that have nothing to do with crime," Rumbaut said. "It's become very controversial."