Mr Trump later followed it up with another post saying: "He is a national disgrace who is destroying the city of London!"

saying现在分词短语是another post的定语，等于another post which said: "He..."

关于say的这种用法，《牛津》词典的释义及例句如下：

[transitive, no passive] (of something that is written or can be seen) to give particular information or instructions

+ speech 

The notice said ‘Keep Out’.

His comments came after five attacks in London in less than 24 hours left three men dead and three others injured.

left在这里是复合宾语及物动词，意为导致、致使，dead 和 injured 都是形容词作宾语补足语，不是后置定语。