The Russian government warned U.S. authorities that Boston Marathon bomber Tamerlan Tsarnaev was a violent radical Islamist more than a year and a half before the April 2013 bombing.
There was no active criminal investigation against the Tsarnaev brothers when the information was conveyed. The treaty regarding criminal investigations was not applicable to the Tsarnaevs. That was just basic exchange of intelligence, which was in the mutual interest of both countries.
Apples and oranges.