Inspector Lestrade enters the dining room. The body lies in the middle of the room. Four other people were in the room when the lights went out and one of them is the murderer. Each makes a statement.

Alice: I know who killed her
Ben: I killed her
Carol: Ben killed her
Donald: It wasn't Ben or Carol.

Enquiries reveal that all the suspects are totally untrustworthy and Lestrade correctly perceives that they are all lying. Whodunit?

That's a bit easy. Suppose instead that Lestrade knows that just one suspect is lying. Then whodunit? (This problem came from David Singmaster).