Russian security forces reported on the incident with a so-called "Black Widow" on Friday. The unnamed woman is thought to be part of the same group that struck Moscow's Domodedovo airport on Monday. She reportedly intended to detonate a suicide belt in a busy area near Red Square on New Year's Eve. It would have been an attack that might have killed hundreds.
Instead, it turns out, a spam SMS message from her cellular provider accidentally set off the device early. The SMS came in just hours before the planned attack, and killer her, but not her two accomplices, who were seen fleeing the scene.
In general the phones are kept turned off until the very last second. In this case, Russian authorities believe that the terrorists were simply lazy or careless.