A movement is rising in the U.S. Senate to put the brakes on the push for immigration overhaul, in light of the slowly emerging details about the foreign-born Boston Marathon bombing suspects.

Sen. Rand Paul, R-Ky., in a letter to Senate Majority Leader Harry Reid, D-Nev., today urged him to reconsider immigration legislation because of the bombings, and called for new hearings and a completely new Senate bill from the bipartisan Gang of Eight's proposal.

"Before Congress moves forward, some important national security questions must be addressed," Paul wrote. "The facts emerging in the Boston Marathon bombing have exposed a weakness in our current system. If we don't use this debate as an opportunity to fix flaws in our current system, flaws made even more evident last week, then we will not be doing our jobs."

He said immigration overhaul should not proceed until "we understand the specific failures of our immigration system."

"Why did the current system allow two individuals to immigrate to the United States from the Chechen Republic in Russia, an area known as a hotbed of Islamic extremism, who then committed acts of terrorism? Were there any safeguards? Could this have been prevented? Does the immigration reform before us address this?"

But Sen. Patrick Leahy, the chairman of the Senate Judiciary Committee, today chastised those trying to draw a connection between the Boston marathon bombings and immigration overhaul, saying opponents of the immigration bill should not be so "cruel" to "exploit" the Boston marathon bombing to their advantage.

"Last week, opponents of comprehensive immigration reform began to exploit the Boston Marathon bombing," Leahy, D-Vt., said at a hearing of the Judiciary Committee today. "I urge restraint in that regard. Refugees and asylum seekers have enriched the fabric of this country from our founding."

At an immigration hearing last week, the first for the Gang of Eight's proposal, Sen. Chuck Grassley, R-Iowa., suggested that attacks by foreign-born suspects should put the brakes on Washington's push for immigration overhaul.

"Given the events of this week, it's important for us to understand the gaps and loopholes in our immigration system," Grassley said at a hearing on immigration Friday. "While we don't yet know the immigration status of people who have terrorized the communities in Massachusetts, when we find out it will help shed light on the weaknesses of our system. How can individuals evade authority and plan such attacks on our soil?"

Opening up the second of the two hearings on the immigration overhaul proposal this morning, Leahy scolded his colleague further, without mentioning him by name.

"Let no one be so cruel as to try to use the heinous acts of these two young men last week to derail the dreams and futures of millions of hardworking people," Leahy said. "The bill before us would serve to strengthen our national security by allowing us to focus our border security and enforcement efforts against those who do us harm, but a nation as strong as ours can welcome the oppressed and persecuted without making compromise in our security."

Grassley responded to Leahy's criticism immediately afterward, defending his comments from Friday.

"When you propose gun legislation, I didn't accuse you of using the Newtown killings as an excuse," Grassley said by way of comparison. "I don't hear any criticism of people when there's 14 people killed in West, Texas, and demanding - taking advantage of that tragedy to warn about more government action to make sure that fertilizer factories are safe. I think we're taking advantage of an opportunity where once in 25 years we deal with immigration to make sure that every base is covered."

Sen. Charles Schumer, D-NY., said at the hearing that Americans will "not be satisfied with delays and impediments towards the bill," using Boston as an "excuse" to delay the bill.

"I say that particularly to those who are pointing to what happened, the terrible tragedy in Boston, as a, I would say, excuse for not doing a bill or delaying it many months or years," Schumer said.

Grassley jumped in interrupting Schumer and yelled, "I never said that, I never said that."

The back and forth between Schumer and Grassley continued, with the chairman banging the gavel to interrupt the rather heated debate, underscoring the emotion surrounding not only the response to Boston but the ever-difficult process of immigration overhaul.