WASHINGTON – A top Republican senator, citing the Boston bombings, warned Friday against rushing ahead with a reform of the country's immigration laws, as concerns about terrorism appeared to revive conservative opposition to the proposal.

"Given the events of this week, it's important for us to understand the gaps and loopholes in our immigration system," Sen. Charles Grassley, R-Iowa, said during the first hearing Friday of a sweeping new immigration bill.

"While we don't yet know the immigration status of the people who have terrorized the communities in Massachusetts, when we find out, it will help shed light on the weaknesses of our system," Grassley said at a Senate Judiciary Committee hearing. "How can individuals evade authorities and plan such attacks on our soil? How can we beef up security checks on people who wish to enter the U.S.? How do we ensure that people who wish to do us harm are not eligible for benefits under the immigration laws, including this new bill before us?"

Grassley's comments were the first overt sign of a potential new hurdle for the immigration plan, which has a narrow window for political consideration. He voiced an attitude that had been percolating among some conservative lawmakers that the bombing at the Boston Marathon provides a reason to limit newcomers to the country.

The comments drew a sharp rebuke from one of the chief architects of the bipartisan Senate immigration proposal.

"I'd like to ask that all of us not jump to conclusions regarding the events in Boston — or try to conflate those events with this legislation," Sen. Charles Schumer, D-N.Y., said during the hearing. "In general, we're a safer country when law enforcement knows who is here."

There was no suggestion that the two suspects, brothers who had lived in Dagestan in southern Russia, had entered the U.S. illegally. And authors of the legislation argued that it would improve U.S. national security because the estimated 11 million people now living here illegally would have to come forward for background checks.

Still, a number of Republicans seized on the events in Boston to raise questions about the existing immigration system and the changes proposed in the new bill. And there were concerns among supporters of the legislation that, even if it turns out that the bombing suspects did not violate U.S. immigration laws, the events could inflame anti-immigrant sentiment just as Congress confronts the already formidable task of getting a far-reaching immigration bill to the president's desk.

A spokesman for Sen. Marco Rubio, R-Fla., one of the principle Senate authors of the legislation, said there were "legitimate policy questions" about whether the immigration system played any role in what happened in Boston.