Zimmerman: Martin's last words were "it's over"

(CBS News) SANFORD, Fla., - The trial of George Zimmerman, the neighborhood watch volunteer charged with murdering Trayvon Martin is still months away, but the investigation continues.

CBS News correspondent Mark Strassmann has come up with new information about what happened that February night when Zimmerman shot the unarmed teenager in a gated community in Sanford, Florida.

At the heart of this case is what happened of the course of 90 seconds: from the moment Zimmerman hung up with a non-emergency dispatcher -- to the first 911 call from a neighbor.

Wednesday new details emerged from sources close to the investigation. A responder at the crime scene told CBS News that he and others saw wounds on the knuckles of one of Martin's hands as he lay dead on this lawn, suggesting Martin had thrown a punch.

Zimmerman told police he fired the shot at point blank range while on his back, as the 17-year-old straddled him.

He also told investigators Martin did not die immediately but mumbled either "it's over" or "you got me."

Special prosecutor Angela Corey's team has built a murder case against Zimmerman on evidence not available to the original state attorney or Sanford police - information that includes the autopsy report on Martin.

State crime lab analysis of his clothes, including gun powder residue that could indicate the distance between of the men.

Cell phone records from Zimmerman that include text messages he sent up to one month after the shooting

There are also two surveillance videos, one from the 7-11 where Martin bought Skittles and an ice tea, the other from the clubhouse in the gated subdivision where Zimmerman shot him.

The special prosecutor's case also includes Zimmerman's medical report the day after the shooting. It listed a broken nose, two black eyes and a cut in the back of the head.

A source also told CBS News an unreleased police report noted Zimmerman's sweatshirt had "grass stains, and was wet on the back."

Details from Trayvon Martin's autopsy show the bullet entered the left side of his chest, and shattered the ventrical, one of his heart's two large chambers but the round did not leave his body.

The reports also noted the fatal wound's surrounded by a two-by-two inch pattern called stippling, caused by gunpower burns. It suggests Zimmerman fired inches away from the teenager.

Mark Strassmann

Mark Strassmann is CBS News' senior national correspondent based in Atlanta. He covers a wide range of stories, including space exploration. Strassmann is also the senior national correspondent for "Face the Nation."