To be sure, U.S. law-enforcement officials say it isn't clear whether knowing the content of the text messages would have changed what the FBI learned in 2011 about Mr. Tsarnaev's turn toward radicalization.

WSJ: U.S.: Russia Withheld Intel on Boston Bomb Suspect

Police secured clear evidence that Noble was using a mobile phone to exchange text messages with a friend.

BBC: A19 death crash texting driver jailed