Thursday's shootings in Tennessee which left four Marines dead at the hands of a Muslim Kuwaiti immigrant have put some on edge over increasingly relaxed legal immigration policies, programs which a number of U.S. senators are working to expand.

The U.S. currently admits about 100,000 Muslim immigrants per year, which is the fastest growing immigrant demographic, according to Breitbart.

According to the Center for Immigration Studies, the immigrant population from predominately Muslim countries has grown from 1.7 million in 2000 to 2.7 million in 2013.

Pew Research estimates that because of lax legal immigration policies, the U.S. Muslim immigrant population will more than double over the next 20 years, from 2.6 million in 2010 to 6.2 million in 2030.

"Unlike illegal immigrants, legal immigrants invited into the United States with green cards are granted automatic work permits, welfare access, and the ability to become voting citizens," writes Breitbart. "The importation of Middle Eastern immigrants through the nation's refugee program has led to the development of pockets of radicalized communities throughout the United States."

Tennessee, where 24-year-old Kuwaiti Muslim immigrant Muhammed Youssef Abdulazees opened fire Thursday on military facilities killing four Marines, happens to have one of the fastest growing immigrant populations in the nation.

Both of Tennessee's senators, Sens. Lamar Alexander and Bob Corker, would like to expand immigration programs, as they helped pass the Gang of Eight bill which would have tripled the number of green cards granted over the next decade.

But as Kenneth Palinkas, head of the U.S. Citizenship and Immigration Services Union, recently warned, the Islamic State group plans to exploit legal immigration policies to gain entry to the country. He listed a number of attacks, such as the Boston Marathon Bombing, that would never have occurred if the federal government didn't have such loose policies.

"It is also essential to warn the public about the threat that ISIS will exploit our loose and lax visa policies to gain entry to the United States. Indeed, as we know from the first World Trade Center bombing in 1993, from the 9/11 terrorist attacks, from the Boston Bombing, from the recent plot to bomb a school and courthouse in Connecticut, and many other lesser-known terror incidents, we are letting terrorists into the United States right through our front door," Palinkas said.

That hasn't stopped two Republican senators, Sen. Pat Toomey of Pennsylvania and Sen. Dan Coats of Indiana, from announcing plans on Thursday to substantially increase immigration from predominately Muslim countries.

Their plan is to "pair highway funding with modernizing our immigration system for highly skilled workers," expanding the number of STEM workers from countries such as Pakistan, India and Saudi Arabia. As Breitbart notes, the Tennessee killed was an engineering student.

"As we saw last Congress, there are forward-thinking bills that would expand opportunities for highly skilled immigrants to come here, grow the economy, create jobs and pay taxes. They are not only good policy, but also a revenue windfall for Washington," the senators wrote. "We should also expand and reform existing visa programs for highly skilled employees, which studies show create more jobs for American workers."

Presidential contender Sen. Marco Rubio, R-Fla., has also introduced immigration legislation that would increase opportunities for Muslim immigrants by establishing an uncapped green card program for foreign STEM students and tripling the number of H-1B visas.