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"Map of Bones" is the second book in the Sigma Force series by James Rollins. The Sigma Force is a special group of operatives in the US government that specialize in getting in and out nasty situations, typically with ancient artifacts. Well, at least they do in the books I've read so far!
"Map of Bones" starts out with a horrific massacre during a midnight mass in Germany. It turns out that the massacre had multiple purposes. One, to distract from the theft of the bones in the reliquary. Two, to thumb the noses of thieves at the Catholic church. And three, as a grisly scientific experiment.
This begins a race across Europe to catch the thieves, who are a part of something called the Dragon Court. Two operatives of Sigma Force team up with a member of the Italian police force, and her uncle, who works in the Vatican. It takes all of their specialized knowledge, and a good bit of luck, to figure it all out as they track down clues that were originally left in the 13th century.