WASHINGTON (Reuters) - U.S. President-elect Donald Trump’s plan to rapidly deport 2 to 3 million illegal immigrants with criminal records would further tax a system already stretched to its limits, current and former U.S. immigration officials say. Immigration courts, which issue deportation orders, set bond and grant or deny asylum, currently have a backlog of more than 500,000 cases.  Boosting staff could help address the problem but that could prove difficult. Officials at the Departments of Justice and Homeland Security say they have had trouble quickly finding and vetting enough qualified candidates to fill all the positions for judges and immigration agents that Congress has authorized.  The Executive Office of Immigration Review, which adjudicates immigration cases, has so far been able to fill only 294 judge positions out of the 374 Congress has authorized because the process is slow by necessity. Candidates for the job “face a rigorous screening process comparable to that of the appointment of a U.S. attorney,” said spokeswoman Kathryn Mattingly.  Trump said last week after his election victory that once he takes office he will move to deport or incarcerate up to 3 million illegal immigrants who have criminal records.     At U.S. Immigration and Customs Enforcement, a force of 6,200 agents responsible for arresting and deporting criminal migrants is already spread thin. While some agents could be redeployed to other areas, more agents would likely need to be hired if deportations were sharply escalated, said an Immigration and Customs Enforcement official who asked not to named. The agency would probably need to add detention space as well, the official added.  More apprehensions of migrants could also strain the Border Patrol, which has struggled to fill open positions. Congress has mandated a force of more than 21,000 border agents, but it currently stands at just over 19,000. Former U.S. Border Patrol chief Mike Fisher said the agency ran into problems a decade ago after Congress asked it to double the number of Border Patrol agents from 6,000 to 12,000 between 2004 and 2006. The push to hire quickly helped fuel corruption, a March 2016 report by the Department of Homeland Security’s independent advisory council found.   The report concluded that the rapid hiring was exploited by Mexican drug gangs, which sent in cartel-friendly applicants for the patrol jobs. Once hired, they then facilitated drug smuggling across the U.S.-Mexico border. The agency began using polygraph tests in 2010 to weed out applicants with criminal histories or ties to cartels. Fisher said the testing made it harder to find qualified candidates, estimating that by the time he left in 2015, the border patrol made one hire out of every 100 applicants. “I can’t imagine if they said ‘double it’ now,” Fisher said.  Trump could try to change laws and procedures to expedite deportation processes, which could reduce the need for additional personnel.  On Friday, Trump tapped Republican Senator Jeff Sessions to be attorney general. Sessions has supported Trump’s plan to build a wall on the U.S. border with Mexico and undo Obama’s executive actions on immigration.  During President Barack Obama’s time in office, the United States has deported about 2.5 million illegal immigrants, more than under any other president. 