Copyright Â© 2010 The Oklahoman Police suspected a pharmacist planted evidence inside the drugstore to bolster his claim he killed a masked robber in self-defense, reports show. "I find it very suspicious,” Oklahoma County District David Prater said of the new evidence, a spent .22-caliber bullet shell casing. The pharmacist, Jerome Ersland, 58, is charged with first-degree murder in a case that touched off a nationwide controversy. His trial is set in June. A security guard reported finding the shell casing in July under a bookcase in a drugstore office. Police detectives asked the guard repeatedly if Ersland could have planted the casing for him to find. "It’s humanly possible,” guard Charles Perkins said, according to police reports on his interview. Ersland fatally shot the robber, Antwun Parker, on May 19 inside the Reliable Discount Pharmacy in south Oklahoma City while another robber fled. Prosecutors charged him with first-degree murder after reviewing a security camera recording of the robbery. The discovery of the shell casing brings back up a key dispute in the case: Did the robbers shoot at the pharmacist? The casing could not have come from Ersland’s two guns. Ersland told police he fired in self-defense because both robbers shot at him. He contends one bullet hit his wrist, breaking his watch. He said he shot Parker again because Parker was getting up after suffering a head wound. Prosecutors contend Ersland was the only one who fired any shots. Prosecutors also say only one robber, Jevontai Ingram, was armed but Ingram did not shoot. Ingram’s gun was never recovered. "We’ve reviewed the video and it doesn’t appear that the weapon was ever shot and there were not any bullet defects in the pharmacy from the suspect’s weapon,” Prater said. Ingram, now 15, has pleaded guilty to first- degree murder for his role in the robbery. He has testified he did not fire the gun. Ersland is charged with first-degree murder because prosecutors allege the pharmacist went too far. They allege he intentionally shot Parker five more times in the abdomen after knocking Parker down and unconscious with a shot to the head. Prosecutors contend Parker was still on his back on the floor when he was killed. Parker was 16. The district attorney said the shell casing will be tested to see whether it has any DNA on it. He said any DNA found on the shell casing would be compared to Ersland’s DNA and to DNA from two men accused of planning the robbery. The district attorney said Ersland could be charged with a crime, such as falsifying evidence, if his DNA matches DNA found on the shell casing. "The technical investigators of the Oklahoma City Police Department swept that whole pharmacy looking for any evidence whatsoever,” Prater said. "That raises my suspicion as to the validity of this evidence. … As in any case, if we find that anyone involved in the case has planted evidence, there are obviously potential legal ramifications for that.” Ersland denied planting the shell casing, saying he is never alone in the pharmacy. "That’s so ridiculous,” he said. "That would be stupid to do that.” The security guard, Perkins, 67, stopped protecting the pharmacy in November. Perkins told The Oklahoman last week, "It is possible he could have put it under there. ... He kept saying, ‘It’s strange that nobody found any of the shell casings from the other gun.’ He repeated that over and over and over.” Perkins told police Ersland "jumped for joy” when he found the casing, according to the police reports. The guard also reported Ersland said of the shell casing, "That’s the missing link.” Perkins told detectives Ersland did not want to report the discovery to police because he said he didn’t trust them. Detectives eventually learned about shell casing from the pharmacy’s owner. The pharmacist’s lead defense attorney, Irven Box, said the value of the shell casing as possible evidence at trial won’t be known until further testing is done. "It’s just another piece in this gigantic puzzle that we’re trying to work through in regards to Mr. Ersland’s case,” Box said. The Oklahoman this month obtained copies of the confidential police reports on the shell casing.