The airline, based in Atlanta, said one passenger was injured after biting into a sandwich but declined medical treatment from paramedics after the plane landed in Minneapolis Sunday, The Wall Street Journal reported.
The sandwiches were served to business-class flyers traveling from Amsterdam to the United States Sunday. Two flights were to Atlanta, one to Minneapolis and one to Seattle.
"Delta is taking this matter extremely seriously and is cooperating with local and federal authorities who are investigating," Delta said in a statement. "Delta has taken immediate action with our in-flight caterer at Amsterdam to ensure the safety and quality of the food we provide on board our aircraft."
The airline said its caterers must "adhere to strict criteria in order to offer our customers the very best onboard meals. The safety and security of our passengers and crew is Delta's No. 1 priority."
A Delta spokeswoman told the newspaper U.S. Customs and Border Protection agents, local law enforcement authorities, paramedics and Delta staffers met the four flights when they landed and the FBI was in contact with Delta headquarters by phone.
The spokeswoman said besides the one passenger who found a needle, flight attendants or federal air marshals discovered objects, including one case in which a needle was found, after a plane had landed, in a sandwich that had not been served, the spokeswoman said.
Delta's food in Amsterdam is provided by Gate Gourmet, which a company spokeswoman said is providing its "complete support" to Delta and law enforcement investigators.