Post Office burglary

After a local Post Office burglary, four suspects were being interviewed. Below is a summary of their statements. The police know that each of them told the truth in one of the statements and lied in the other. From this information, who committed the crime?
Alan said:
It wasn't Derek
It wasn't Brian

Brian said: It wasn't Charles
It was Derek

Charles said:
It was Alan
It wasn't Derek

Derek said:
It was Charles
It wasn't Alan