The Devil's Teardrop

The Devil's Teardrop by Jeffery Deaver

From the Goodreads website:

"After an early morning machine-gun attack by a madman called the Digger leaves dozens dead in the Washington, D.C., subway, the mayor’s office receives a message demanding twenty million dollars by midnight or more innocents will die. It is New Year’s Eve, and with the ransom note as the only evidence, Special Agent Margaret Lukas calls upon retired FBI agent and the nation’s premier document examiner Parker Kincaid to join the manhunt for the Digger --- or for hundreds, the first moments of the new year will be their last on earth."

Gotta say I really liked this one. There were a couple of twists that totally surprised me and had me going right to the last few pages.

I also have to say that I really love Goodreads. It lets me keep track of what I've read and what I want to read.