Sanford, Fla. - They heard "yelps" and "howling."

The witnesses giving wrenching testimony in a crowded courtroom here Wednesday were using those words to describe the sounds they heard in the final minutes of a 17-year-old boy's life.

The contest to answer the basic questions - Who yelped? Who howled? - animates the courtroom drama now deep into its third day of testimony in this central Florida city near Orlando.

Defense attorneys want to portray George Zimmerman - the neighborhood watch volunteer accused of second-degree murder in the shooting of Trayvon Martin - as the victim. He was the one calling out in pain, they'd like jurors to believe, and he only shot Martin to defend himself.

Prosecutors are portraying Zimmerman - who was 28 at the time - as the aggressor, as an overzealous wannabe police officer who shot Martin not because he had to, but "because he wanted to."

A neighbor, Jayne Surdyka, dabbed tears from her eyes on the witness stand Wednesday morning while she listened to a recording of a 911 call she placed on the night in February 2012, when Martin was shot in the heart by Zimmerman.

"Sorry, I'm just shaking. . . . I'm shaking. I can't help it," she said in a voice wobbly with emotion. She sobbed and said over and over, "Oh, my God."

Surdyka told rapt jurors how she knelt at her window - at the "cat ledge" where her pet often perched - and saw two figures "wrestling" in the darkness. She heard voices, too - "angry" voices, "agitated" voices. One was "loud, dominant," the other "higher pitched," a voice that sounded to her more like a boy than a man. And she heard a "yelp."

Another neighbor, Jeannee Manalo, told jurors that she heard "rumblings . . . struggling . . . howling."

The testimony of the two prosecution witnesses lent emotional power to the state's case, which has been buttressed this week by graphic crime scene photos and the display of Martin's now-famous "hoodie" sweatshirt singed by Zimmerman's bullet.

But on a day that might have been an unambiguous success for the prosecutors, defense attorneys were still able to peck holes in the government case. Under persistent cross-examination by lawyer Mark O'Mara, Manalo acknowledged that she was relying on an old photograph of Zimmerman - when he was much younger and skinnier - to support her statement that he was on top of Martin while they wrestled near her home. Defense attorneys have said Martin was on top of Zimmerman and pounding his head into the ground.

And Surdyka may have given the defense an opening to sow doubt when she recounted hearing three gunshots - "pop, pop, pop." Only a single shot was fired, according to evidence in the case.

The issue of race hovers over the proceedings. Civil rights activists have claimed that Zimmerman, who has white and Hispanic parents, targeted Martin because he was African-American. Prosecutors were able to overcome defense objections and play recordings Wednesday of 911 calls that Zimmerman made in the past to report African-Americans he suspected of entering his neighborhood to commit crimes.

As the tapes played, jurors leaned forward, quickly scratching out notes on legal pads. There was one fewer of them than the day before. An alternate juror - a man who described himself as an arm-wrestling enthusiast and was identified only as Juror B72 - was dismissed for an undisclosed reason that Judge Debra Nelson said had nothing to do with the case.