How did nobody suspect Scar killed Mufasa?

They are lions. Not very smart. They don't even know how to use computers.

They were convinced Simba was dead without ever seeing a corpse. And when he came back, they all believed he had killed Mufasa when he was still a cub. Definitely not the brightest creatures.

How? The official story by Scar was that Simba was in the area where the Stampede was. Mufasa of course jumped to help his son and died in the process. Simba was also assumed to be dead. When Simba came back, everyone assumed the only reason for him being away was because he was ashamed of killing Mufasa. I don't remember anyone (except Scar's loyals) assuming more as he killed Mufasa by accident. If he had come back, Scar could have gone just along with the story, as Simba was also convinced of his guilt. Simba just had an accident before he had become grown up.