build a case
To compile evidence that supports an argument or charges against someone or something. The job of the prosecutor is to build a case to prove that the defendant is guilty. We're trying to build a case because many employees have been wrongly terminated by the company.
build a case (against someone)
and gather a case (against someone)to put together the evidence needed to make a legal or disciplinary case against someone. The police easily built a case against the drunken driver. As soon as we gather the case against her, we can obtain a warrant to arrest her.