the thieves dropped the torch and ran away as quickly as the

the thieves dropped the torch and ran away as quickly as they could.不理解后半句的结构.
the thieves dropped the torch and ran away as quickly as they could.
这句话主要是后半句不理解,书上说as quickly as they could是状语,是什么状语?注释上说前一个as是副词,后一个 as是连词,引导比较状语从句,could后省略了run.
这句到底是什么结构,是固定结构吗,另外they could,这个could我怎么也看不明白为什么用could.这个could当什么讲呢,是can的过去式,还是语气词,如果是语气词是不是就不考虑时态问题了.
谁能讲一个后半句的句型和本句中could的用法.