Interpol Wang returned to the Bureau after investigating a murder scene. The guard Lao Zhang asked him what the result was. Wang said he knew that the murderer had been to the scene. Lao Zhang knew that Interpol Wang was teasing him, because he didn't need Wang to tell him at all. He knew that if someone was a murderer, he must be at the scene at the time of the crime.
If Wang Wu was not at the scene of the crime, he would not be a murderer can be inferred according to the answer.