The Boston Marathon bombings that killed three people and injured more than 170, the shootout that followed on the campus of Massachusetts Institute of Technology and a massive explosion in a fertilizer plant in Texas made up a black week for the United States and highlighted domestic security issues.

Police later identified two brothers, Tamerlan Tsarnaev and Dzhokar Tsarnaev, ethnic Chechens who migrated to the US more than a decade ago, as suspects in the Boston bombings. Tamerlan was later killed in a shootout with police and Dzhokar was arrested after being cornered in a boat in the Watertown area of Boston.

The injured suspect has told interrogators that he and his brother acted alone and without the help of any foreign terrorist group, and were driven by hard-line Islamist views and anger over the US wars in Afghanistan and Iraq, US officials said Tuesday.