Des Moines police say the murder victim found Tuesday afternoon in the Des Moines River was a 13-year old middle school student. Sergeant Bruce Elrod says Linda Williamson was last seen 11:30 Monday night. He says the young girl was a special education student who led a tough life that ended too soon.Elrod says they’ve determined Williamson drowned, but could not reveal the exact details of how it happened.He says they’re still trying to determine if Williamson was sexually assaulted.Elrod says there are no obvious signs of sexual abuse, but police are waiting for lab results to confirm there was no abuse. Elrod says police are trying to get the man who found and reported the body to come in for questioning. Elrod says the man is reluctant to come in because he told dispatchers there is a warrant out for his arrest. Elrod says the man may have some key information. A bike path runs next to the river in the area where the body was found. Elrod is asking anyone who may’ve been in the area Tuesday to call police.Elrod says Williamson lived with her mother and stepfather three to four blocks away from the site where her body was found. He says it was not unusual for her to be out on her own.