Squinting in the midday sun, Teri Knight scans the curtains of green fields that border this highway in search of clues that could lead her to a terrible place: the gravesite of her slain children.

She believes her son and daughter are buried out there, somewhere among tens of thousands of anonymous acres. Maybe here in Illinois, maybe another state. For two years, she has been haunted by the same question: Where?

In July 2003, authorities say, Knight's ex-husband shot and killed their daughter, Sarah, 14, and son, Philip, 11. Manuel Gehring told police in New Hampshire that he wrapped his children in plastic and placed duct-tape crosses on their chests. Then, he said, he dug a shallow grave for them somewhere along this concrete artery than runs through the heart of middle America.

Police drove Gehring along Interstate 80 shortly after the killings to look for the L-shaped grave. But he could not find it. Neither could law enforcement officers or volunteers who searched a 650-mile stretch from Pennsylvania to Ohio to Indiana to Illinois to Iowa -- the area where he is believed to have buried the children.

Last week -- just a few days after the two-year anniversary of her children's disappearance -- Knight came to look.

She retraced Gehring's path along I-80, but the purpose of her six-day search was to raise public awareness, rather than an attempt to do the seemingly undoable: find an unmarked burial site.

"I would love to say that I could have some instinct and we'd drive right to some place and find them," she said. "But I can't leave my hopes there. I have to be realistic."

So Knight and her second husband, Jim, drove the route and displayed this sign on the passenger side: "Help Us Find The Kids, philipandsarah.com."

They stopped at places where Gehring had been, left fliers at convenience stations and scouted fields that fit the general description he had provided police. And they wore plastic badges near their hearts, each with a photo of the two smiling children.

Knight hopes word of her search will pique the public's attention and produce a tip that will allow her to bring her children home.

"I need to do this," she said. "What mother wouldn't want their children buried near her or want to know [where] their final resting place is? It's just what they deserve."

Gehring had drawn a rough sketch of the burial site and said it was off an I-80 exit in the Midwest amid tall grass. A six-foot-high metal fence, an old water pump, a yellow or tan commercial building, firewood, large slabs of concrete, five or six large trash-filled concrete construction cylinders, a pile of white or gray rocks and willow-like trees were nearby.

Those general clues would be all he would provide. Gehring, 44, committed suicide in jail in February 2004 while awaiting trial.

Teri and Jim Knight stop in Iowa City during their search for Teri's daughter and son, who are believed to be buried somewhere along Interstate 80 in middle America. Her ex-husband killed the children in 2003.