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Homeland Security Secretary Janet Napolitano cancelled a planned appearance before the immigration-reform bill hearing at the Senate Judiciary Committee Friday to oversee her agency's participation in the manhunt in Boston. The suspects: Chechen immigrants.

"They got their start as refugees. Refugees from war," the uncle of the Boston Marathon suspects said of the young men, one of whom was slain in a shootout with police in Watertown, Mass., overnight.

The unfolding events in Boston and its surrounding suburbs immediately began to influence the debate over the immigration bill under discussion in Washington, as conservatives on Twitter began to prick at Sen. Marco Rubio over his support of immigration reform and to call for a slow-down in efforts to create a pathway to citizenship for undocumented immigrants already in the Unites States.

"Given the events of this week it's important to understand the gaps and loopholes" in the immigration system, Sen. Chuck Grassley of Iowa, the ranking Republican member on the Senate Judiciary Committee, said in opening remarks at the hearing. He said hoped the hearing would "shed light on the weaknesses of our system" and "how can we beef up security checks," as well as how to insure that "those who would do us harm do not receive benefits under the immigration laws."