MIAMI — At least 121 killings within a four-year span were carried out by convicted immigrants who were not deported, according to a 2015 U.S. Senate Judiciary Committee document recently reviewed by el Nuevo Herald.

Every year, federal immigration authorities release foreign nationals convicted of crimes — including murder — both because the U.S. Supreme Court has prohibited indefinite detention or because their countries refuse to take them back even after immigration judges have ordered deportation.

While the release of convicted immigrant criminals has been routine since the Supreme Court issued its landmark ruling 15 years ago, the practice is now in the national spotlight because President Donald Trump has made it imperative to deport immigrant convicts as quickly as possible lest they commit more crimes.

Research by the U.S. Senate Judiciary Committee has elicited evidence that could be used to back Trump's claim. A committee document, obtained by el Nuevo Herald, contains comprehensive information from the Department of Homeland Security about the number of immigrant convicts in the United States, their whereabouts, whether immigration authorities have succeeded in deporting them and whether they committed additional crimes after being released.

A committee letter sent to the Department of Justice and the Departments of State and Homeland Security nearly two years ago said that at least 121 homicides "could have been avoided" between 2010 and 2014 had Immigration and Customs Enforcement, under the Obama administration, deported immigrant convicts instead of releasing them.

"This disturbing fact follows ICE's admission that, of the 36,007 criminal aliens it released from ICE custody in Fiscal Year 2013, 1,000 have been re-convicted of additional crimes in the short time since their release," according to the letter, dated June 12, 2015.

The Senate Judiciary Committee letter revealed that 121 immigrant convicts were charged with homicide following their release from ICE custody between 2010 and 2014. It also noted that in 2014, ICE released 2,457 immigrant convicts because of the Supreme Court ruling prohibiting detention of deportable foreign nationals beyond six months.

Most of these immigrant convicts are nationals of 23 countries described by ICE as "recalcitrant" because they routinely refuse to take back deportables. The bulk of these immigrant convicts in 2014 — 1,183 — were from Cuba, according to the letter. The other "recalcitrant" countries include Afghanistan, Algeria, China, Iran, Iraq, Libya, Somalia and Zimbabwe, according to ICE.

Generally, foreign nationals who are convicted of crimes are put into deportation proceedings after they complete their sentences.

A committee spokeswoman did not provide additional information on the letter when contacted by el Nuevo Herald last week.

But in response to the letter, Sarah Saldana, then-director of ICE, stated that 33 of the 121 immigrant convicts accused of "homicide-related offenses" had been released on bond at the discretion of immigration courts. Another 24 were released because ICE was unable to obtain approval to deport them to their countries within the 180-day deadline set by the Supreme Court in 2001.

Immigrant rights advocates, for their part, argue that blaming immigrants for a surge in crime is not supported by studies. The Washington-based American Immigration Council in 2015 published an in-depth report about the chief findings in those studies.

"For more than a century, innumerable studies have confirmed two simple yet powerful truths about the relationship between immigration and crime," according to the AIC report. "Immigrants are less likely to commit serious crimes or be behind bars than the native-born, and high rates of immigration are associated with lower rates of violent crime and property crime. This holds true for both legal immigrants and the unauthorized, regardless of their country of origin or level of education."

One weapon Trump has suggested he could wield to compel countries to take back their deportable nationals is halting the issuance of visas to visitors and immigrants from those nations. While this has yet to happen, Trump has shown a willingness to strike harshly through executive orders like his decision to bar admission of refugees for 120 days and nationals from seven Muslim-majority nations for 60 days, a measure that took effect last weekend at international airports, seaports and border ports of entry — and at foreign airports where many travelers were prevented from boarding U.S.-bound flights. The measure sparked chaos at airports around the nation and has since been lifted because of a court order that is being challenged by the Trump administration.

In a speech in Phoenix during the campaign, Trump vowed to deport immigrant criminals regardless of whether their countries agreed to take them back.

"There are at least 23 countries that refuse to take their people back after they've been ordered to leave the United States," Trump said. "Including large numbers of violent criminals. They won't take them back. So we say, 'Okay, we'll keep them.' Not going to happen with me, not going to happen with me."

While Trump has not himself articulated a threat to deny visas to nationals of these countries, he has suggested that he might take such a course of action.

Trump's executive order temporarily halting the worldwide refugee program and the entry of nationals from the seven Muslim-majority countries contains language stipulating that if the countries from where those people came do not provide certain requested information, then the president will prohibit the entry of nationals from those countries.

If those countries refuse to provide the information Washington wants, then the secretary of state will deliver to Trump a list of countries "recommended for inclusion on a presidential proclamation that would prohibit the entry of foreign nationals . from countries that do not provide the information requested," according to the executive order.

Denying any future visas from countries reluctant to take back deportable nationals has long been cited as a weapon to induce compliance but has never been widely used.

Former DHS Secretary Jeh Johnson told the Senate Judiciary Committee in April 2015 that he did not believe visa sanctions against recalcitrant countries was the right policy.

"I don't necessarily believe that we ought to suspend immigration, travel from any of these countries because of this particular issue," Johnson said at the time.

The letter from the Senate Judiciary Committee notes that the United States can "promote compliance" with ICE deportation efforts by denying diplomatic visas or any other types of visas to nationals of "recalcitrant countries that deny or delay accepting the return of one or more aliens."

The letter also noted that Cuba has been "perpetually on ICE's list of recalcitrant countries because it refuses to take back" deportable Cubans, except the 2,746 mentioned in a list agreed to by Washington and Havana in 1984.

On Jan. 12, when then-President Barack Obama revoked the "wet-foot/dry-foot" policy, American officials indicated that the new policy allowed for the return and deportation of undocumented Cubans who arrived after it took effect. They were nebulous as to whether the new policy will apply to the more than 35,000 Cuban convicts who have been ordered deported after 1984.

On the issue of convicted immigrants, the committee's letter focused on the 121 prisoners released between 2010 and 2014 because they were charged with homicide after they were allowed to walk out of jail.

This is a significant issue because one of the pillars of Trump's opposition to immigrants with criminal records is that some have been linked to murders of American citizens.

Trump has cited examples of his claim in various speeches.

In his Phoenix immigration speech Aug. 31, Trump recalled the case of 21-year-old Sarah Root of Des Moines, Iowa, whose death in January 2016 was blamed on an undocumented immigrant who had been released after having been in custody despite being in the country illegally and having failed to show up in court for prior alleged crimes. Eswin Mejia, a Honduran, was charged in connection with Root's death in a vehicle crash, was freed on bail and fled.

Trump also cited the case of Grant Ronnebeck, a 21-year-old convenience store clerk from Mesa, Ariz., whose slaying was also blamed on an undocumented immigrant who had been previously convicted of burglary and had been released from federal custody.

Officials said Ronnebeck was killed over a pack of cigarettes in January 2015, and the slaying was linked to Apolinar Altamirano, a Mexican national who was in deportation proceedings but who had been released on bond by an immigration court judge.

A third case Trump mentioned was that of Kate Steinle, gunned down in San Francisco. An undocumented Mexican, Juan Francisco Lopez Sanchez, who had been deported five times previously but had managed to cross the border again undetected, is charged in the slaying.

ICE officials have told Congress that they tried to prevent Lopez Sanchez from bonding out of jail but failed to do so because authorities in San Francisco, a so-called "sanctuary city," ignored an immigration detainer for the defendant.