WASHINGTON (Reuters) - The chief of U.S. Immigration and Customs Enforcement (ICE) said on Thursday her agency released 19,723 undocumented immigrants last year who had been convicted of crimes, drawing a volley of criticism from Republican lawmakers. In a heated exchange at a congressional hearing into crime committed by illegal immigrants, Republicans grilled top Obama administration immigration official Sarah Saldana.   The issue is at the heart of Republican demands for tighter control of U.S. borders and has often featured in rhetoric over immigration in the 2016 presidential election campaign.     Saldana defended her record and pleaded for comprehensive immigration reform.     “I cannot tell you how disheartening it is to sit here and hear a very important issue related to the topic of immigration reform be bandied about as a political football,” she said. Republicans on a House oversight committee blamed ICE for releasing undocumented immigrants from jail whom they said went on to commit homicides, sexual assaults and drunk driving offenses.   The topic of released illegal immigrants who commit later crimes gained attention on the campaign trail last year after Kathryn Steinle, 32, was shot and killed in San Francisco by Juan Lopez-Sanchez, an undocumented immigrant and convicted felon who had previously been deported to Mexico five times.  Donald Trump, the front-runner for the Republican presidential nomination, said in July that Steinle’s death showed the need for tighter control of the U.S.-Mexico border.  ICE decides whether to deport undocumented immigrants. The Obama administration has a policy of deporting immigrants who pose a violent threat, but it releases some who have served prison sentences for their crimes and are deemed not dangerous, Saldana said.  House oversight committee Chairman Jason Chaffetz said the agency’s practice was “infuriating.” When undocumented immigrants have served their jail time but cannot be repatriated, immigration authorities have no choice but to release them, she said.  Only specific crimes require mandatory jailing of immigrants, Saldana said, adding that Congress should consider changing that policy.  Countries such as Syria, Iraq, Afghanistan, Haiti and Guinea have refused to take back some of their citizens after they have been ordered deported from the United States, Saldana said. Saldana said she is meeting with the State Department to persuade them to enforce visa restrictions on countries who do not take back their citizens. 
Republicans, Obama immigration chief clash over criminal immigrants