Sen. Rand Paul, R-Ky., in a letter on Monday to Senate Majority Leader Harry Reid, D-Nev., urged him to delay the immigration reform process until lawmakers understand how two Chechens, accused in the Boston Marathon bombings, were allowed legal status here.

Paul said the Boston bombings highlight a national security concern that lawmakers must address in the just-begun process of debating new immigration reform that would affect the status of as many as 11 million undocumented people.

A bipartisan group of senators, known as the Gang of Eight, last week introduced an immigration bill. Authorities identified two brothers, Dzhokhar Tsarnaev, 19, and Tamerlan Tsarnaev, 26, as suspects in the twin explosions at the Boston Marathon. The older brother died in a shootout with police, and the younger brother has been captured. Both men had legal immigration status.

“I believe that any real comprehensive immigration reform must implement strong national security protections,” Paul said in his letter. “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.

“We should not proceed until we understand the specific failures of our immigration system,” the letter continued. “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?”

The Kentucky Republican, who is believed to be a prospective 2016 presidential candidate, is calling for hearings and for reforms to closely scrutinize individuals from “high-risk areas of the world.”

Read the full text of Paul's letter.