Undoubtedly others are more knowledgeable than I about the incident. I just recall that it almost turned into a fistfight. This is one description:
"Frazier is taken to Dallas Police headquarters and placed in a small interrogation room for more than six hours. He is interrogated for hours at a time by two different teams of detectives who insist that he’s a killer, although there is nothing in his past to remotely suggest that he could be capable of committing such a crime. Frazier doesn’t have so much as a traffic ticket on his record. Frazier holds firm and denies the charges. Finally, Captain Will Fritz, frustrated by Frazier’s resolve, comes into the interrogation room with a typewritten confession and demands that Frazier sign in it, without even reading it. Frazier refuses, causing Fritz to lose his temper. When Fritz tries to hit him, Frazier parries the blow and prepares to defend himself. At that point, Fritz throws up his hands in frustration and leaves the room. At around 3 a.m. the next morning, police let Frazier go."
If that's accurate, it would certainly have rattled me when I was 19 years old!
(There does seem to be remarkably little documentation or discussion of the incident, so hopefully someone will know more than I do.)