WASHINGTON (CNN) -- On the surface, the type of heated exchanges that boiled over at a congressional hearing on Monday on immigration reform appeared to center on the two foreign-born suspects in last week's Boston Marathon bombings.

Beneath the surface, however, simmered tensions over the depth, breadth and pacing of plans to overhaul U.S. immigration policy currently wending through the Senate.

"Last week, opponents of comprehensive immigration reform began to exploit the Boston Marathon bombing. ... I urge restraint in that regard," Senate Judiciary Committee Chairman Patrick Leahy, D-Vermont, said at the beginning of Monday's hearing. "Refugees and asylum-seekers have enriched the fabric of this country from our founding."

Iowa Republican Sen. Chuck Grassley, the top Republican on the Judiciary Committee, was one of the first conservatives to publicly connect the bombings to the roiling immigration debate and question whether the attacks would suggest a need to slow down and re-examine immigration reform efforts.

He took exception to suggestions that he and others are trying to "exploit" the bombing for political purposes.

"I don't hear any criticism ... when there (were) 14 people killed in West,Texas, and (some political activists took) advantage of that tragedy to warn about more government action to make sure that fertilizer factories are safe," Grassley said.

Both sides in the contentious immigration debate accuse the other of cynicism and using a national tragedy to score political points.

The back and forth focuses on Tamerlan and Dzhokhar Tsarnaev, brothers of northern Caucasus origin who lived legally in the United States.

Investigators believe the pair were behind the attacks near last Monday's Boston Marathon finish line that killed three people and injured more than 170 others.

Dzhokhar Tsarnaev, 19, immigrated to the United States with his parents in 2002 and became a U.S. citizen in 2012. Tamerlan Tsarnaev, 26, followed a few years later and was a legal resident.

Supporters of the bipartisan immigration measure authored by the so-called "Gang of Eight" accuse the other side of using any excuse, including using the Boston bombings, to stoke nationalistic sentiments as a way to derail the bill.

New York Democrat Sen. Charles Schumer, one of the "Gang of Eight" senators who drafted the immigration plan, said on CNN's "State of the Union" some on the right who opposed the bill "from the get-go" are simply using Boston "as an excuse" to stall the legislation.

Opponents assert that the measure's supporters are rushing through immigration reform legislation.

Republican Sen. Dan Coats of Indiana said Sunday on ABC's "This Week" that Congress should wait until the emotional reaction to the violence subsides before tackling immigration legislation.

Other lawmakers such as Rep. Louie Gohmert, R-Texas and Republican Rep. Steve King of Iowa have made similar assertion.

Immigration reformers seek to avoid deja vu

Immigration law experts say they can see little merit to attempts to connect the Boston Marathon bombings to current immigration reform efforts.

"What I do see is the opponents of immigration reform who have no arguments about the morality and politics of (this immigration package) very cynically jumping on this horrible tragedy to derail this bill," said David Leopold, an immigration attorney and past president of the Washington-based American Immigration Lawyers Association.

However, others caution that the current immigration reform measure has "fundamental problems" and applaud efforts to slow the pace of legislation.

"The bill is 132,000 words. The New Testament is 180,000 words. This isn't quite as long but it's going to take a little time to go through," said Steven Camarota, director of research for the non-profit Center for Immigration Studies.

CNN's Tom Cohen and Ashley Killough contributed to this report.