New Ground - S3-E19
Question: Why does the guy who was interrogating SG 1 think they know how the dead man they found was killed? Why does he think they're lying? Why doesn't SG 1 tell their interrogator that they weren't there when the man was killed?
Answer: The planet had two groups of people that were basically at war with each other. The guy interrogating SG-1 was part of the Bedrosian military and thought SG-1 was working with, or part of, the Optricans (their enemy). He simply doesn't trust his enemy and thinks they're responsible, and he certainly isn't going to listen to reason. Plus, he's trying to preserve his religious beliefs and doesn't want to think his people came through the Stargate.
Answer: He knows that SG-1 didn't kill the man because they were in custody when he died. He suspected that there was a fourth member (Teal'c) and was trying to get them to admit it.