Six months after business class travelers on several Delta Air Lines flights from Amsterdam found needles in sandwiches served in flight, an investigative body in Amsterdam has been unable to find the perpetrator.

The investigation began after the needle-tainted turkey sandwiches were found in July on two flights from Amsterdam to Atlanta, one to Minneapolis and one to Seattle.

The office responsible for investigating criminal offenses at Amsterdam’s Schiphol Airport has completed its investigation “for the moment,” but would continue investigating if any new information arises, according to Natasja Keijzer, a spokesperson for the public prosecution office.

The investigation included questioning employees who delivered the sandwiches and managers and watching security camera videos. But, “we didn’t manage to locate the person who is responsible for the needles in the sandwiches,” Keijzer said in an e-mail.

Atlanta-based Delta said after the incident, it “took immediate action” with its in-flight caterer in Amsterdam to ensure food safety and quality, revised security procedures, and cooperated with investigators.

Airline caterer Gate Gourmet, which made the sandwiches, said it has adjusted protocols and will continue to work closely with regulators.