US airline Delta is trying to determine how needles about one inch long, got into in-flight meals served on at least four flights from Amsterdam to the United States.

A Delta spokeswoman confirmed on Tuesday that the needles were found Sunday in six sandwiches on four flights. Passengers discovered four of them.

The FBI and the airport's police department have opened criminal investigations.

The sandwiches were prepared in Amsterdam's Schiphol Airport. The flights affected included one to Minneapolis, one to Seattle and two to Atlanta.

The sandwiches were made by Gate Gourmet, one of the world's largest airline caterers, with facilities on five continents.

The company serves many airlines but only Delta flights appeared to be affected. The company said it was investigating without elaborating further.

Kristin Baur, Delta Air Lines spokeswoman, said security has been stepped up at all of the Gate Gourmet facilities used by the airline.

Trenton Higareda, Aviation Security Specialist, says airline food is not screened in the same way as passengers or their luggage.