The people did nothing but complain about both of them. First they came to Jesus with an evil intention. They were afraid to say that John's baptism was from men because they were cowards. They were afraid of the people who considered John a prophet. One sin leads to another sin.
Sin is endless.
Now comes a third sin. They did know. But their own sins of denial and their fears of the people caused them to lie. They are of their father, the devil, the father of lies. There is a further sin which is not stated here but which is implied.