Evidence of more police incompetence

Can someone please explain the behaviour of Thai police in this case?
The alleged culprits are caught. They accepted they bought these body parts and were sending the parcel by DHL. The police interrogated them and let them go because “the two Americans were very cooperative during interrogation”.
The duo is said to have fled to Cambodia but the police say “the pair promised to return to Bangkok for further questioning”!
(The sun will rise from the West when this happens.)
Now the police learn the duo has a criminal background. Why didn’t did they check with the FBI the day they were caught?
The address on the package belongs to a company owned by one of the culprits. So much for their story that they were sending these body parts as a prank to their friend in the US.
Am I missing something here?
Rakesh Sodhia