The fact that they and their families were granted political asylum to be in the US is troubling ... This can only reinforce the xenophobic nature of the ongoing immigration discourse in this country, most likely negatively.

Apart from any "sleeper agents" theories just the payback is enough to cause second thoughts.

Being named Tamerlan is a name in honor of Timur AKA:Tamerlane. So, the older brother had delusions of reaching his namesake's glory?

This seems a wannabe terrorist act of some very deranged individuals toward their host country and little more than that ... The surviving brother should be tried as a mass murderer.

see: 18 USC § 2332f - Bombings of places of public use, government facilities, public transportation systems and infrastructure facilities