>> reporter: more than a decade after the worst terrorist attack in our nation's history, it seems the u.s. immigration system that allowed the september 11th hijackers to enter the kunly legally is still very flawed. some lawmakers question whether tamerlan tsarnaev should have been so easily admitted back into the u.s. after his trip to russia, chechnya, and dagestan last year. after all, in 2011, the russian government warned both the fbi and the cia that they were worried that tamerlan had become an extremist and would be traveling to meet with underground groups. the fbi investigated and found nothing, but should immigration officials have been told more? now another concern. three friends of alleged bomber dzhokhar tsarnaev have now been arrested for their alleged involvement after the boston marathon attacks. 19-year-old dias kadyrbayev, azamat tazhayakov, and robel phillipos all went to school with tsarnaev at the university of massachusetts, dartmouth. two are accused of removing