DETECTION : Killer in The Case of Honest Suspects

What was the case?

Let's first see what are the statements made by 4 suspects.

Joplin: I did not kill Handel. Either Grieg is the killer or none of us is.

Grieg: I did not kill Handel. Gershwin is the killer.

Strauss: I did not kill Handel. Grieg is lying when he says Gershwin is the killer.

Gershwin: I did not kill Handel. If Joplin did not kill him, then Grieg did. 

ANALYSIS :

1] If Joplin is the killer then his first statement would be lie & other statement must be true. But his second statement contradicts assumption that he is killer. So Joplin can't be the killer.

2] Let's assume Grieg is the killer. His first statement must be lie and second must be true. His second statement points to Gershwin as a killer. But there is one killer among 4 so Grieg and Gershwin can't be killers together. Therefore, Grieg can't be the killer.

3] Suppose Gershwin is the killer. Again, his first statement is lie and second is true. His true second statement suggests that Grieg is the killer as Joplin is assumed to be innocent in the case. Again, both Grieg and Gershwin can't be killers together since there is only one killer. Hence, Gershwin is not the killer.

4] Therefore, Strauss must be the killer. His first statement is lie and second statement is true. And that's how Grieg is lying when he is saying Gershwin is the killer.

CONCLUSION : 

STRAUSS is the one who committed the crime and his first statement is lie and second is true.

The first statements of rest of all suspects are true and second statements of each are lie.