Airports of Thailand (AoT) chairman Sita Divari confirmed after talks with Immigration Bureau police on Monday morning that the man - the accused leader of a gang thieves in Germany - escaped from custody and disappeared into greater Bangkok after arriving on May 15.
Kohl remains at large, and is believed to be hiding somewhere in greater Bangkok.
"What we know now is that Kohl has left the airport," said Suvitchpol Imjairat, chief of the Immigration Bureau at the airport.
Meanwhile, the Airports of Thailand (AoT) has raised concerns over Kohl's getaway. "The incident is an important lesson for us that requires serious checks for possible security flaws," Mr Sita said Monday.
The investigation found Kohl had disconnected the electricity supply at an emergency exit in a restricted area in the G concourse.
The fugitive was identified as Carlo Konstantin Kohl by Pol Maj Gen Suvitchpol, after a meeting at Suvarnabhumi several hours hours later.
Pol Maj Gen Suvitchpol said Kohl illegally left the airport after managing to bypass immigration.
The Australian police accompanying him had gone to sleep in a transit room at the airport after arriving on May 15. Their connecting flight to Germany had been delayed. When they awoke, they discovered he was no longer in their custody, he said.
It remains unclear whether Mr Kohl fled on the night of May 15 or the early hours of May 16.
Kohl was being taken from Brisbane to Frankfurt via Bangkok on Thai Airways International flights.
The flight from Brisbane arrived at Suvarnabhumi about 8pm on May 15. The connecting flight was supposed to leave for Germany at 11pm, but was delayed because of bad weather in Europe and rescheduled for 8am on May 16.
Kohl's Australian police escort went to sleep in the transit lounge.
"Mr Kohl has left the airport. He did not pass through the immigration desks," the airport immigration police chief said.
"He faces a charge of illegally entering the country and investigators of the Immigration Bureau are trying to locate him."
He said Thai authorities should not be blamed for the fugitive's escape. Australian police had not alerted Thai immigration authorities that they were escorting a prisoner through Suvarnabhumi airport en route to Europe.
Pol Maj Gen Suvitchpol said the German had completed a jail term in Australia and, under Australian law, he was being accompanied by Australian authorities back to his country to face outstanding charges there.
Kohl has a history of drug, robbery and violent crimes committed in Germany, and the Australian states of Queensland and Victoria.
Kohl was in Australia living a "lawless life" for more than two years on a tourist visa before he was arrested in 2010.
Australian media reports said he was a drug runner, and "was also breaking into homes up and down the (Sunshine) Coast" in Queensland.
A second media report carried details of Kohl's drug dealing. He "went on a crime spree to raise $25,000 to cover a drug debt he incurred by misplacing about 1,000 ecstasy tablets he had been trafficking," said The Telegraph of Brisbane.
He was jailed for nine years when he faced Brisbane Supreme Court last September. Justice Jean Dalton said the level of criminality was high and Kohl had committed a "shocking series of offences".
Defence barrister Sarah Thompson said her client went "off the rails" when his father died in 2006.
Kohl became eligible for parole in January because of the time he had already served.
Read the original story here.