AMSTERDAM—Police at Amsterdam's Schiphol Airport say they still do not know how needles got into turkey sandwiches on Delta Air Lines flights from Amsterdam to the United States, but are investigating.

Airport Police spokesman Robert van Kapel said Tuesday Dutch detectives are working on the case together with the airline and U.S. law enforcement authorities.

Delta says what appear to be sewing needles were found in five sandwiches on Sunday. One passenger on a flight to Minneapolis was injured.

The other needles were on two flights to Atlanta and one to Seattle.

The sandwiches were made in the Amsterdam kitchen of catering company Gate Gourmet.