DHS may have known of the trip because Tamerlan admitted it when he appeared before Immigration and Customs inspectors at the port of entry on his re-entry to the US. Such a long absence is suspicious in and of itself for any green card holder, but apparently the DHS quietly re-admitted him to the US without any further action.
This is life under Big Sis the frat girl.
Younger FReepers might like to know that L.H. Oswald later went on to become the lone assassin of President JF Kennedy.
File under - Things That are Weird