During so-called "open tests" conducted in February, nine of the 12 banned objects which EU authorities tried to smuggle through airport security were not found, the Rheinische Post reported on Monday.
In undercover testing meanwhile they were able to to smuggle six of twelve weapons or bomb parts through the x-ray units without the security personnel noticing.
The results were seen by public broadcaster WDR but are considered highly secret and will not be published.
Kötter Aviation, the company responsible for the security checks at the airport, said in a statement on Friday that its employees passed 98 percent of tests conducted by the German federal police.
“In this respect were are surprised at the results of the tests conducted by the EU inspectors at Cologne-Bonn airport,” said managing director Klaus Wedekind.
Wedekind said he had taken immediate action in light of the test results to further train his employees but states that success in tests carried out by the police showed “the very high performance of our employees.”
German airports don't have a spotless record when it comes to scrutiny from the EU over security arrangements.
In May 2015 the European Commission started legal proceedings against Germany for not adequately checking security measures at its airports. In July, the interior ministry conceded that “quality control checks weren't carried out to the necessary extent or with the necessary regularity,”
At the end of 2014, meanwhile, EU inspectors found gaps in security procedures at several airports including Frankfurt, the country's largest air hub.
Authorities were able to successfully smuggle dangerous objects through security on half their attempts at that time, and cited poor levels of training among security personnel as the principal cause.