"Carl Mueller, second from left, sits next to his wife, Marsha, as they honor their daughter Kayla with a candlelight vigil in Prescott, Arizona, on Wednesday, February 18. Kayla, a 26-year-old American aid worker, <a href=""http://www.cnn.com/2015/02/10/world/isis-hostage-mueller/index.html"" target=""_blank"">was killed by the militant group ISIS.</a> She had been captured in northern Syria in 2013."
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