SYCAMORE, Ill. -- A man charged in the 2010 slaying of a Northern Illinois University freshman pleaded guilty Wednesday and received a 37-year prison sentence.

William Curl, 36, formally agreed to a plea agreement in the killing of Antinette "Toni" Keller of Plainfield, Ill., at a hearing at the DeKalb County Courthouse in Sycamore.

Curl was scheduled to go on trial April 11 in the slaying that shook NIU's campus. The university was placed on heightened alert as police searched for Keller - and later her killer, after badly burned human remains subsequently identified as hers were found in a DeKalb park soon after she disappeared.

"We spent two years, pushing and pulling, trying to get to a trial date, and now it's gone and into a plea. It just happened very, very fast for us. We're all in shock," Mary Tarling, Keller's cousin and spokeswoman for the family, said Tuesday after word of the agreement was announced. "That's making it harder to say what we think of the details of the plea, because we just can't even believe we have one. We had no idea we were going to go this route until (Monday)."

In a statement released Wednesday, State's Attorney Richard Schmack defended the plea agreement.

"While we believe a jury would have convicted him of murder following a trial, his sentence could have ranged from 20 to 60 years, and had he only been convicted of arson and acquitted of the other charges, he would have been out of prison before Thanksgiving," Schmack said.

Curl had been charged with first-degree murder, criminal sexual assault and arson in connection with Keller's death.

Keller, an 18-year-old art student, disappeared Oct. 14, 2010, after telling friends she planned to visit Prairie Park to do some sketching.

When she failed to return by the next morning, her friends reported her missing.

A day later, police found human remains near various items consistent with some of Keller's belongings, but they waited seven days before telling the community, drawing frustration from students.

It would take nearly three months - and DNA testing - to identify the remains as Keller's.

Sixteen days after Keller disappeared, Curl was charged.

Police initially interviewed Curl because he frequented the park. When he didn't show up for a second interview, they began hunting for him.

Authorities arrested him in Louisiana after tracking him to Mexico through his credit card use. He was found at a motel after recrossing the U.S.-Mexico border in search of work as a day laborer, officials said.

Prosecutors said his 37-year sentence was to be served in its entirety.

"This office believes it to be the longest sentence imposed at 100 percent on a negotiated plea to a non-death-penalty-eligible murder in DeKalb County, since at least 1980," the state's attorney's office said in a statement Tuesday.

Tarling said the family was still coming to grips with the sentence and called reaction "mixed."

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