Indonesia ? The U.S. Navy has picked up signals from the flight data recorders of an Indonesian jetliner that crashed into the sea on New Year’s Day with 102 people onboard, the U.S. Embassy said Thursday.

The USNS Mary Sears located signals “on the same frequency of the black boxes associated with the missing airplane,” a statement issued by the U.S. Embassy in Jakarta said.

Data from the flight recorders will be crucial in determining the cause of the crash, but retrieving them from the ocean floor at a depth of more than a mile will likely be a difficult, expensive and lengthy operation.

The Adam Air Boeing 737 went missing more than three weeks ago after reporting heavy winds off the western coast of Sulawesi while flying from Indonesia’s main island of Java.