Keep in mind that you can drive all day in that area and, so long as you stay off the major routes and away from towns, never see a single police car.
Say the suspects drove for up to 60 miles before stopping to bury her body. That's an area of 11,309.7 square miles. It sounds like it took 1000 volunteers to search a 10 square mile area. Doing that for the entire area would mean 1,130.97 searches of that magnitude. Assuming each searcher put in 5 hours on the original search, that's 5,654,850 man hours.
On average, workers in the US put in 2,150 hours per year. So to search that area would take 2,630 people working full time for a year to search the entire area.
Granted, you can probably assume that they went somewhere east judging from the official areas to search, so you can cut all the above numbers in half. But even cut in half, that just doesn't seem that do-able for me on a formal basis.
What might be do-able would be a mailing campaign with a flyer including a postage-paid post card to every landowner of record in the area, explaining what happened and asking them to fill in and send back the postage card once they have checked their own treelines. That would cut down considerably on the area left to search.
For the landowners who don't return the postcards, then have a central organisation make contact with them personally, asking them to either search or grant permission for a search by volunteers/LE.
Just an idea.
