When a suspicious package was identified at Windhoek airport in Namibia on Thursday, the flight was halted, the Munich-bound passengers delayed and news sped round the world that an x-ray scanner had been found with batteries attached by wires to a detonator and a ticking clock.

Today, however, a German government minister revealed the bizarre truth: the bomb was fake, manufactured in the US to test airport security. It was not yet clear who had planted "test suitcase", the German interior minister, Thomas de Mazière, said, but the one fact they had established was that the device had been manufactured by a US company that specialises in alarm systems. At no time were passengers' lives in danger.

"This company is a manufacturer of alarm and detection systems and these real test suitcases are built to test security measures," he said.

The US transportation security administration (TSA) confirmed today that it was working with the German and Namibian authorities "to determine the origin of the device and the reason it was being transported on the plane".

According to tests by the German federal criminal police, the suitcase did not contain explosives.

German security experts said yesterday it was most likely that either US or African authorities were behind the test, following the discovery of several parcel bombs sent from Yemen to the US.

The suitcase, which contained batteries connected to a detonator and a ticking clock, was intercepted by authorities at Windhoek airport on Wednesday night and sparked an international terror alert.

The Munich-bound Air Berlin plane on which it was believed the suitcase was due to be loaded was delayed for about eight hours while security checks were carried out. Passengers were questioned by police when the plane landed at Munich airport yesterday morning.

Initial theories that the bomb might have been a dummy run by terrorists were dismissed by US security sources who told German television such "dry runs" were not typical.

Security services and airport scanner manufacturers are known to run a variety of false alarm checks to ensure operators and systems are capable of intercepting likely forms of explosive devices. One leading manufacturer said today that its scanning machines were programmed to generate occasional false positives in order to keep staff alert. The TSA is reported to have organised several series of tests using undercover agents to put dummy bombs through security scanners at US airports. One such test, in 2007, was said to have resulted in 75% of the fake bomb parts passing through unobserved.

Even more embarrassing was a security survey carried out by Slovakian border officials earlier this year using real explosives. Eight pieces of contraband were planted on unwitting passengers leaving on a flight from Bratislava bound for Dublin. The 85g (3oz) charge of RDX slipped into a 49-year-old electrician's bag was not removed by security checks and the passenger flew to Ireland unaware of what was in his luggage. Slovakian officials eventually had to contact police in Dublin to explain the error.

The Windhoek discovery had no impact on the German security alert issued by de Mazière on Wednesday, which related to "concrete information" about a planned Islamist attack on Germany towards the end of this month.

Security measures have been tightened across the country. Extra armed police and military personnel have been sent to airports, railway stations, seaports and luxury hotels.

Several incidents of unattended suitcases and packages found at railway stations in Berlin today proved to be harmless, but the atmosphere in public places remained jittery.

With Germany's Christmas markets, a magnet for tourists from around the world, due to open on Monday, all police leave has been cancelled.