Further doubt emerged Thursday over the circumstances of Florida teen Trayvon Martin's murder, as the mother of a young witness said her son was "pressured" by police and another onlooker cast doubt on killer George Zimmerman's claims.

An unnamed witness speaking on CNN's "Anderson Cooper 360" said the entirety of the scuffle he saw between the two took place on grass, challenging Zimmerman's claim that Martin had smashed his head against a sidewalk after punching him in the face -- causing him to shoot the 17-year-old dead in self-defense.

In reference to the size difference between 28-year-old Zimmerman and Martin, the witness described the fight as being between a "larger man" and a "boy." While the witness said he was not able to see who was on top of the other as the two scuffled, he said "there wasn't a lot of movement" immediately before he heard a gunshot.

Zimmerman "didn't appear hurt" as he walked away, he added, and he didn't see any blood on him.

Meanwhile, the mother of a 13-year-old who also witnessed the killing told the New York Daily News that when Sanford police interviewed her son eight days after the incident, they "pressured" him to give more detail than he recalled.

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    The youngster, Austin Brown, had been referenced by police as having seen Martin on top of Zimmerman during the fight, but his mother, Cheryl, said he saw only one person lying on the grass and could not identify them.

    "He kept telling them he couldn't see anything because it was too dark," she said. "He said he couldn't see the race or anything. He never saw a second person.

    "Then they asked him if he saw what the man was wearing. They gave him a multiple choice question and gave him three colors. He said, 'I think it was red.' Knowing my son, I believe he felt pressured to give the color."

    It was not clear whether she was present for her son's police interview.

    Martin was walking from a 7-Eleven to the home where he was staying with his father in a gated community outside Orlando on Feb. 26 when Zimmerman, a neighborhood watch volunteer, phoned 911 to report that the black teen, who was wearing a hoodie, looked suspicious.

    Despite being told by a police dispatcher not to follow Martin, Zimmerman left his vehicle to go after him, leading to their fatal exchange. Martin was unarmed, carrying only a bag of Skittles and a can of iced tea.

    The new witness accounts came a day after video surfaced showing Zimmerman in police custody after their deadly clash, but without obvious signs of the head injuries he claims were inflicted upon him in the scuffle.

    A lawyer representing Martin's parents, Benjamin Crump, said the footage was "the smoking gun" for those seeking to have him arrested.