George Zimmerman\'s attorney will rest his case Wednesday after a U.S. forensics expert said Trayvon Martin was likely on top of Zimmerman when he was killed. The defense expected to wrap up its evidence introduction before the end of the day, defense attorney Mark O\'Mara told Circuit Court Judge Debra Nelson in Sanford, Fla. At that point, the state will have an opportunity to call rebuttal witnesses. The case could go to the jury Thursday or Friday, the Orlando Sentinel reported. Zimmerman, 29, is charged with second-degree murder in the Feb. 26, 2012, shooting death of Martin, an unarmed black 17-year-old. Zimmerman pleaded not guilty, saying he shot Martin in self-defense after Martin knocked him to the ground, punched him in the nose, straddled him on the ground and repeatedly slammed his head into the concrete pavement. The prosecution maintains Zimmerman, a volunteer neighborhood watch coordinator, pursued Martin and instigated the confrontation that ended in Martin\'s death. Jurors Tuesday heard from a widely known forensic pathologist who testified the evidence indicates Martin was probably on top of Zimmerman, leaning over him, when he was shot. Dr. Vincent Di Maio, a retired chief medical examiner for Bexar County, Texas, based in San Antonio, testified for the defense the different-size circles of gunpowder around the bullet hole in Martin\'s chest and the hole on his hoodie and shirt indicated the clothes were hanging 2 to 4 inches from Martin\'s chest. The gunpowder marks around the bullet hole in Martin\'s chest measured 2 inches in diameter, whereas the ring on Martin\'s hoodie and shirt was much smaller, Di Maio said, explaining they would have been the same size if the clothes were up against Martin\'s body. \"This is consistent with Mr. Zimmerman\'s account, that Mr. Martin was over him, leaning forward, at the time that he was shot,\" Di Maio said. Under cross-examination, Di Maio said it was possible Martin could have been trying to get off Zimmerman, even if he was leaning. Di Maio also told the jury he was paid $2,400 so far by Zimmerman\'s attorneys to testify at the trial, adding, \"This is not exactly a complicated case forensically.\" The bullet Zimmerman fired entered Martin\'s heart from the front, in a left-to-right direction, and plunged into one of his lungs, Di Maio testified. After he was shot, Martin\'s heart likely kept beating for 1 to 3 minutes, and Martin could have moved or talked for at least 10 to 15 seconds after he was shot because of the reservoir of oxygen in his brain, Di Maio said. Di Maio said he also evaluated Zimmerman\'s injuries and told jurors the defendant suffered at least six blows to the head and should have gone to the hospital.