WASHINGTON -- Emotions erupted in a contentious Senate hearing on immigration reform Monday during a heated exchange over the Boston bombing.Opponents of a bipartisan Senate immigration bill said the Boston Marathon tragedy raised questions about the current system and urged restraint in moving ahead with an overhaul of the nation's immigration laws.But Senate Judiciary Committee Chairman Patrick Leahy, a Vermont Democrat, urged lawmakers to refrain from exploiting the Boston tragedy for legislative gain on an issue that impacts millions of employers, workers and immigrants."Let no one be so cruel as to use the heinous acts of two young men last week to derail the dreams and futures of millions of hard-working people," Leahy said.The ranking Republican, Sen. Charles Grassley of Iowa, said last week that the bombing showed it was "important to understand the gaps and loopholes in our immigration system. This is not something to be rushed.Sen. Charles Schumer, D-N.Y., an author of the bill, issued a rebuke Monday, saying the tragedy in Boston should not be used "as an excuse for not doing a bill or delaying it many months or years."That prompted an outburst by Grassley: "I never said that! I never said that!"Schumer said his comments were not directed at Grassley, or any individual lawmaker, but toward sentiment to delay proceedings on the bill. He said specific measures in the bill could "make a Boston less likely."The two immigrant Chechen brothers believed responsible for the attacks were in this country legally, and one became a naturalized U.S. citizen in 2012.