One person was injured in the incident but declined treatment on arrival in Minneapolis.
The needles were discovered in food on planes as they were en route on Sunday from Schiphol Airport in the Netherlands to Minneapolis, Seattle and two flights to Atlanta, according to Delta spokeswoman Kristin Baur.
Two of the needles were found by passengers, she said. The American carrier has told all 18 flights from Amsterdam to stop serving the sandwiches.
Two more needles were discovered, including one found by a federal air marshal, CNN reported.
Federal agents have begun a criminal investigation, said Special Agent Stephen Emmett, an FBI spokesman in Atlanta.
Gate Gourmet, which provided prepared sandwiches to Delta, said the sandwiches originated at the firm's facility in Amsterdam.
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"This is a terribly upsetting situation," said Gate Gourmet spokeswoman Christina Ulosevich. "First and foremost is the safety of the traveling public. There's nothing more important to us at all than the safety of the passengers and crews."
Delta is now serving sealed prepackaged food on these flights instead of the sandwiches, and no other needles have been found, Baur said.
The Transportation Security Administration says it notified all US airlines with flights from Schiphol to the United States of the findings.
"TSA continues to closely monitor the review of the incidents as well as the security protocols being conducted by the air carrier and the airport authority," spokesman David Castelveter said.
Delta says it has been in contact with the FBI, authorities in the Netherlands and Gate Gourmet and is fully cooperating with them in this investigation.