MINNEAPOLIS -- Delta Air Lines Inc. is trying to figure out how needles got into turkey sandwiches on four flights from Amsterdam to the U.S.

The airline said that what appear to be sewing needles were found in six sandwiches on four flights on Sunday. One passenger on a flight to Minneapolis was injured, but the passenger declined to get medical attention, according to Delta spokeswoman Kristin Baur. 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, and were to be served to business class passengers on Delta flights.

After the needles were found, passengers got pizza instead.

Baur said security has been increased in its food preparation process, and it is using more prepackaged food.

"Delta is taking this matter extremely seriously and is cooperating with local and federal authorities who are investigating the incident. Delta has taken immediate action with our in-flight caterer at Amsterdam to ensure the safety and quality of the food we provide onboard our aircraft," the airline said in a written statement.

Gate Gourmet spokeswoman Christina Ulosevich said the company has gotten no reports of similar incidents on any of the other airlines it serves out of Amsterdam. She said the company did not yet know how the needles got into the sandwiches.

Gate Gourmet issued a statement saying, "We take this matter very seriously, and we have launched our own full-scale investigation." It also said it was "heightening our already stringent safety and security procedures, to prevent any recurrence."