For reasons unknown, Russian intelligence was wiretapping a telephone call between Tamerlan Tsarnaev and his mother in 2011. And whatever they overheard troubled them enough to ask the U.S. to investigate him. It may have to do with a six-month visit Tamerlan made to Russia.
The conversations are significant because, had they been revealed earlier, there might have been enough evidence for the FBI to initiate a more thorough investigation of the Boston bombing suspects' family.
A mystery man known only as "Misha" could be key. He is reportedly a conservative Muslim convert who influenced Tamerlan and may have radicalized him.
Investigators are walking back through the lives of Tamerlan and his younger brother Dzhokhar, trying to find out what set them on the path to terrorism.
U.S. officials increasingly suspect the brothers had technical assistance in making their pressure cooker bombs. They do note, however, that there is no evidence to suggest that the brothers had a wider terror plot.
Another question mark is the Tsarnaev brothers' mother, Russian-born Zubeidat Tsarnaeva. She was put on a U.S. terror watch list in 2011. She has denied any involvement in terrorism.
Dzhokhar, the surviving suspect, remains in a federal prison hospital outside Boston. He has reportedly stopped answering questions.
Meanwhile, more victims of the bombing blasts are leaving Boston hospitals. Area medical centers say dozens of patients who suffered shrapnel wounds from the bombs are being discharged.
In all, 260 people were injured.
(The Associated Press contributed to this story)