There are only three possible answers. The FBI discounted the Russian intelligence information. The FBI was inept in their initial questioning and background investigation. Or the hypersensitivity of the Obama administration policy of not being accused of Islamophobia filtered down into the ranks of the FBI investigative policy. Most likely all three played a part in the FBI's failure to keep Tamerian Tsarnaev under investigation.
It is time to stop with this political correctness policy of not profiling Muslims for fear of being called racist. We are at war with