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Rogelio V. Solis, AP Photo
Patricia Unger, mother of slain mayoral candidate Marco McMillian listens as the family's attorney calls for a federal investigation into McMillian's death, amid their complaints that local officials are withholding information about the case, at a news conference in Clarksdale, Miss., Thursday, May 9, 2013.

JACKSON, Miss. — An attorney for a slain Mississippi mayoral candidate's relatives says the sheriff's department has mishandled evidence and withheld information about its investigation.

Attorney Daryl Parks also demanded a federal investigation during a news conference Thursday outside the family home of late Clarksdale mayoral candidate Marco McMillian.

The FBI says it's monitoring the investigation. Parks wants agents actively involved.

McMillian's body was found Feb. 27. The case drew national attention after his campaign said he was the first viable, openly gay political candidate in Mississippi.

Lawrence Reed is charged with murder. Authorities say Reed crashed McMillian's SUV the day before the body was discovered.

Parks says investigators mishandled the SUV by releasing it from custody and leaving it unprotected at a tow yard. He says it may have transported McMillian's body.