The Boston Marathon bombing and the odor of corruption

The more the public learns of Tamerlan Tsarnaev the less it is apt to trust the FBI. The story that the media has parroted, that Russia twice warned the FBI of Tsarnaev's terrorist propensities but somehow he fell through the cracks may be about to unravel. Lawyers for Dzhokhar Tsarnaev claim that the FBI tried to make the older Tsarnaev an informant on the Chechen and Muslim community. The government has responded that it has “no evidence” of that.
In a court filing, lawyers for Dzhokhar requested evidence on Tamerlan from the prosecution. They claimed this information could help with Dzhokhar’s defense. in that filing one reads;

While it is unlikely that Dzhokhar Tsarnaev was the direct target of surveillance
or searches, a report by the House Homeland Security Committee released on March 26
suggests that government agents monitored Tamerlan and his communications during
2011 and perhaps 2012.

So the FBI was monitoring Tamerlan Tsarnaev and his communications while he was plotting the Boston Marathon bombing?
Why was the bombing not stopped? Was the FBI also aware of the triple homicide supposedly committed by Tamerlan Tsarnaev and Ibragim Todashev which they were questioning Todashev when he was shot to death? The FBI’s relationship with Tamerlan Tsarnaev , the monitoring of his communications is a major red flag and should be investigated by public congressional hearings.