Parts of Cologne airport have been evacuated after a "breach" involving a woman who was able to walk through security without being checked.
At least one flight has been cancelled and several more are delayed after the failure to check a passenger sparked a major police operation.
A spokesman for Cologne Bonn Airport told The Independent the evacuation of Terminal 2 was still going on and a "federal police investigation" had been launched.
But he denied earlier reports that both terminals were evacuated, or that there had been a separate incident involving "unauthorised people" entering the airfield.
"A woman passed security check without being checked," he said. "In such cases there is a rule that the whole security area has to be evacuated."
Passenger Marco Tarsia posted an image to social media showing throngs of passengers being forced to leave the terminal in order for checks to be carried out again.
Airport officials were unable to say how long the investigation would take when contacted at midday local time, but monitoring site AirLive reported that passengers of at least one Ryanair flight, bound for Berlin, had been forced to leave the plane and return to security screening.
The breach - and subsequent police operation - come amid a climate of heightened checks and concerns for safety following the Paris attacks and Russian plane bombing in November last year.
Such threats to European security have seen the establishment of a joint continent-wide counter-terror initiative, the European Counter Terrorism Centre (ECTC), launched at the end of January.
And security fears linked to the refugee crisis have become an issue in Cologne in particular, following a spate of sexual attacks on women by men of North African and Arab origin on New Year's Eve.
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