Delta Air Lines Inc. has added security to its meal production as the FBI and Dutch police investigate the sewing needles found in sandwiches on four of the carrier’s July 15 flights bound for the U.S. from Amsterdam, according to an article from Bloomberg.

Criminal inquiries have been opened into the matter after one person was injured on a flight to Minneapolis and single needles in sandwiches were found in two flights to Atlanta and one flight to Seattle.

According to a statement from Delta spokeswoman Kristin Baur, the security around meal production in Amsterdam has been tightened, and the amount of pre-packaged food served to passengers has increased. In an emailed statement, she also added that Delta is requiring all kitchens to “increase already rigorous security while the investigation is under way until we determine the cause.”

According to Bloomberg, Delta removed items prepared at the Gate Gourmet facility in Amsterdam and replaced them with pre-packaged foods.

No other incidents were reported on flights from Amsterdam to the U.S. on July 15 or yesterday, Delta has said.