ISTANBUL (AP) — A New York City woman who went missing and was later found dead in Istanbul had suffered a fatal blow to the head, police said Sunday.

Forensic experts have not concluded their autopsy report on the victim, Sarai Sierra, but it is "clear" the head injury caused her death, said Istanbul police chief Huseyin Capkin.

NTV, a Turkish broadcaster, says 15 people have been detained for questioning in the case.

Sierra, a 33-year-old mother of two, was last heard from on Jan. 21, the day she was to fly home from a vacation. Her body was discovered Saturday evening near the remnants of ancient city walls.

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 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.