William Curl, 34, of DeKalb, Ill., is charged with first-degree murder, criminal sexual assault and arson, DeKalb County State's Attorney John Farrell said during a press conference late Friday night.
Police say Keller did not know the suspect.
Curl was extradited to Illinois Friday and was being held on $5 million bail at the DeKalb County Jail. He was taken into custody in Covington, La., by the U.S. Marshalls on charges of obstruction of justice and unlawful possession of a motor vehicle.
Investigators say Curl was a frequent visitor to Prairie Park and on October 14th he encountered 18-year-old Keller.
"This was a gruesome murder and we believe this was a crime of opportunity...they were in a secluded area and we know that she had walked to the park and he was in the park and found her," said Chief Bill Feithen, DeKalb Police Dept.
Police say Curl took his mother's car and went to Mexico, but he was arrested in Louisiana after he returned. U.S. Marshals picked him up Tuesday at a hotel.
Sources close to the investigation say Curl came to the attention of the DeKalb police when he called them about a fire in the woods at Prairie Park, where Keller told friends she was going to work on an art project.
Keller disappeared October 14. Police discovered burned remains days later in Prairie Park near the NIU campus. Items thought to be Keller's were found by the remains.
"One simply cannot imagine the anuish they have been through have endured the last two weeks. The family has been truly an insprition to everyone involved in this investigation through their support and quiet determination to see this investigation through to completion," said Farrell.
"It's shocking just to start with seeing a face. And you just kind of look at that face and you're just trying to reconcile the whole thing and fight the flood of emotions all over the map that come at you," said Mary Tarling, Keller's cousin.
Police say they are still building their case against Curl and they want people to call in with any information about him or the case.
Curl is eligible for the death penalty. He is scheduled to appear in court on Saturday.