A New York City woman found dead after vanishing while vacationing in Istanbul suffered a fatal blow to the head, officials say. Turkish forensic experts said Sunday though they have finished their not autopsy report on 33-year-old Sarai Sierra, it is "clear" the head injury caused her death. The report comes as family and friends of the mother of two gathered for a special prayer service at the Manhattan Grace Tabernacle Church in East Harlem where the woman's family once attended. "Everybody got together, we prayed for the family, for their loved ones, for their suffering," attendee Ruben Gonzalez told MyFoxNY.com. Turkish media says 15 people have been detained for questioning in the case Sierra was last heard from on Jan. 21, the day she was to fly home from a vacation. In Washington, the U.S. State Department confirmed Sierra's death in Istanbul, thanked Turkey's government for its extensive efforts to locate her body and said the investigation of what happened to her would continue. "We are also appreciative of the many expressions of sympathy received from the Turkish people," the State Department said in its statement. Sierra, whose children are 9 and 11, had left for Istanbul on Jan. 7 to explore her photography hobby. "She had an awesome smile, she was very upbeat, very sarcastic, she was a real city girl and I'll never forget her," friend Nina Rivera told MyFoxNY.com. She was in regular contact with friends and relatives, and had told them she would visit Galata Bridge, which spans Istanbul's Golden Horn waterway, to take photos. "There's anger, there's questions that we have," Rivera said. Click for more from MyFoxNY.com.