While Russia did alert U.S. authorities about Tsarnaev’s possible extremism, it kept out some facts, namely text messages referencing his desire to join a militant group, the source said.

However, sources told the Wall Street Journal that the United States also likely would have withheld such details for fear of divulging intelligence sources and methods.

In the texts, Tsarnaev wrote to his mother about his interest in joining the militant movement carrying out attacks against Russia in the Caucasus region, the law enforcement source told CNN.

The Russians did not pass these texts on to American officials when they passed the original intelligence about Tsarnaev, the source said.