The FBI is investigating how sewing needles got into turkey sandwiches served on four U.S.-bound Delta Air Lines flights out of Amsterdam Sunday -- an incident that has raised concerns about the security of in-flight food and its potential to be used in an act of terrorism.

Passengers discovered the needles in sandwiches on flights from the Netherlands to Seattle, Minneapolis and Atlanta, according to Delta. At least two passengers on the Minneapolis-bound flight suffered minor injuries and one passenger, James Tonjes, is said to be on the drug Truvada, an antiretroviral drug recently approved by the FDA to treat HIV.

Delta ordered all 18 flights from Amsterdam to immediately stop serving the sandwiches and provide passengers with pizza instead. The Transportation Security Administration says it then notified all U.S. airlines with flights out of Schiphol Airport of the findings and continued to monitor the situation for additional developments.

The FBI's Atlanta office, meanwhile, has opened up a criminal investigation into the incident and is coordinating their efforts with Dutch detectives.

Delta spokeswoman Kristin Baur said the airline is taking the matter extremely seriously and is cooperating with local and federal authorities.

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, she said in an emailed statement.

Baur added that the airline requires its caterers to adhere to strict criteria and that safety and security is its No. 1 priority. The airline, Baur noted, will serve more pre-packaged food on flights out of Amsterdam until the situation is resolved.

The tainted sandwiches originated at Gate Gourmet's facility in the Schiphol Airport, and spokesman David Fisher said the company has received no reports of tampered products on any other airlines it serves out of Amsterdam.

We take this matter very seriously, he said. Gate Gourmet immediately launched a full investigation to determine the root cause of this disturbing incident, and we are treating this as a criminal act.

Gate Gourmet, a subsidiary of Swiss-based gategroup Holding AG, is one of the largest independent global providers of airline passenger products and services.

Fisher said the company is cooperating fully with investigations by local and federal authorities and has increased security in its Netherlands facility, but it added that further details of this matter must remain confidential.

Gate Gourmet operates in 28 countries and serves an average of 9,700 flights each day and 300 million passengers annually. With such a wide reach, the mid-air scare raises concerns about a potential loophole in airport security.

Last year, a local television station in Atlanta performed an investigation at the Gate Gourmet facility at Hartsfield-Jackson International Airport, the world's busiest airport by passenger traffic. The station, WSB-TV, released a video that allegedly shows just how easy it is to add unauthorized items to unaccompanied food carts.

Though catering operations are tightly regulated by food safety authorities who look for things like salmonella, sabotage is often much less of a priority. TSA spokesperson Lisa Farbstein, however, said that the Aviation and Transportation Security Act requires steps be taken to ensure aircraft catering services security.

TSA has developed procedures that airlines and their contractors must follow to ensure the secure movement of catering supplies, carts and vehicles, she noted, adding that the agency conducts ongoing inspections to ensure airlines and contractors comply with security requirements.

As for the alleged infractions in Atlanta, Farbstein claims TSA has inspected Gate Gourmet operations at Hartsfield-Jackson Atlanta International Airport on approximately 20 separate occasions since October 2011, a significant increase over the prior year's period. No violation of security regulations has been found.

Nevertheless, Sunday's incident proves that, for all the security measures that take place at passenger checkpoints, catering operations have the potential to be a gaping hole in aviation security.

Fbi