The investigation into the Boston Marathon bombing yielded two revelations Tuesday: The surviving suspect said he and his brother were inspired by jihadists online, and the brothers bought two large pyrotechnic devices that could shed light on how they built their explosives.

Authorities have been trying to piece together how and why the brothers accused of the April 15 bombing carried out their plans. In limited communication with federal investigators, including handwritten answers, surviving suspect Dzhokhar Tsarnaev said he and his older brother acted alone and without the help of any terrorist group, U.S. officials said Tuesday.

The two were acting as jihadists, motivated by Muslim religious anger at the U.S., the surviving brother told interrogators, these officials said.

Dzhokhar Tsarnaev told investigators that the brothers were self-trained and self-indoctrinated, and were striking back at the U.S. for killing Muslims. His statements and other evidence have led U.S. authorities so far to believe that the bombers weren't part of a larger terror group, though no firm conclusions have been made.

Tamerlan Tsarnaev bought the two pyrotechnic devices in February from a New Hampshire fireworks store, a development being examined by federal investigators probing how the brothers constructed bombs without detection.