The trial of George Zimmerman, accused of second-degree murder in the case of Trayvon Martin, advanced this week with the testimony of Chris Serino and Doris Singleton. Serino, Sanford police detective, and Singleton, another investigator in the case, testified Monday on their investigation following Martin’s death.

Zimmerman has admitted that he shot and killed black teenager Trayvon Martin, but claims that he did so in self defense after the teen attacked him and began banging his head against the concrete. Martin’s family and prosecutors are trying to prove that Zimmerman had ill will against the teen, had followed and scared him, and attacked him partially on the basis of race.

When talking about the teen, Zimmerman answered questions in a straightforward, anger-free way. The state claims he profiled the teen when he saw him walking about the neighborhood’s streets, but according to Zimmerman it was he who was attacked and not the other way around.

Just a few hours after the attack, Zimmerman was quoted in an interview with Serino, saying, “In Catholic religion, it’s always wrong to kill someone.”

Serino responded, “If what you’re telling me is true, I don’t think that what God meant was that you couldn’t save your own life.”

A few days later, though Serino and Singleton suggested that Zimmerman may have been following the teen around before the attack, possibly scaring the teen. That possibility is one that many are taking into account.

“I think that the idea that Trayvon Martin is being followed by George Zimmerman is coming across,” said legal analyst Kendall Coffey in a “Spinning the Law” segment. “And by the way, that’s consistent with other evidence, including the tone of some of the police statements that have not been brought into evidence yet.”

One crucial piece of evidence is the 911 call recording, on which you can hear someone calling for help. Zimmerman claims he had just given the police dispatcher location information on where he had seen Martin and was heading back to his car when he was attacked and began screaming for help. The prosecution, however, claims that the voice calling for help was actually Martin’s.

The sound on the recording has thus far not been enough to conclusively determine who had been yelling for help, but if investigators are able to figure that out, it would be crucial to the case’s outcome—because it could show who the aggressor actually was.