This is the 13634th most frequent Spanish word.
retenidos
"Retenidos" means "detained" or "held back" in English.
In this sentence, 'retenidos' indicates something being withheld or kept back, specifically the official documents at customs.
Los documentos oficiales fueron retenidos en la aduana.
The official documents were withheld at customs.
Here, 'retenidos' refers to individuals being held or kept in a location temporarily, namely passengers on the plane.
Los pasajeros estuvieron retenidos en el avión debido al mal tiempo.
The passengers were held in the plane due to bad weather.
In this usage, 'retenidos' conveys the sense of being detained or kept under surveillance by authority figures, in this case, criminals by the police.