Airport operations were stopped and security areas closed due to the incident that happened early on Friday morning.
Later in the morning, operations resumed but passengers still faced delays at security.
An updated statement on the airport's website said: "The police operation has been completed. Normal operations have resumed.
"There are currently longer waiting times. The airport apologises for the inconvenience and asks affected passengers for their understanding."
Police have not released further details on the nature of the incident.
According to local broadcaster WDR, newly introduced security technology in the passenger control area failed to function properly.
Employees are believed to have identified suspicious objects in a bag, which were recognised as potentially dangerous on the control monitors.
However, instead of the system flagging up the bag and separating it for further inspection, the hand luggage was mistakenly sent back to the passenger.
The passenger then took the bag and disappeared into the airport. Authorities are understood to have searched for the passenger and the luggage so it could be checked.
It comes after disruption to air travellers in Germany on Thursday during an all-day strike by Lufthansa pilots and crew.
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