Police are trying to determine which of the knives self-styled prophet Brian Mitchell allegedly used to abduct the girl last June, said the source, who spoke on condition of anonymity.
The source said Mitchell kept a knife at each of several hidden camps, moving among them to avoid thousands of volunteers who searched the Wasatch foothills for Elizabeth, now 15. The knives were recovered after the girl pointed out the makeshift camps -- some recognizable only by a fire ring -- from a helicopter last weekend.
At one camp investigators found a hole covered with boards. The source said they believe Elizabeth, Mitchell and his wife hid in the hole to shield themselves from helicopters equipped with heat-detecting sensors.
The discovery of the hole was reported Friday in The Salt Lake Tribune.
Mitchell has been accused of taking Elizabeth at knifepoint from her bedroom. He and his wife, Wanda Barzee, were charged Tuesday with aggravated kidnapping and aggravated sexual assault, among other charges.
Elizabeth, Mitchell and Barzee were found March 12 walking in the Salt Lake suburb of Sandy. Dressed in robes, the trio was returning from a winter in San Diego.
Mitchell and Barzee are being held on $10 million bail, and are expected to enter pleas April 1.
Prosecutors said the couple held Elizabeth for the two months after her abduction with little food or water at the network of camps, scattered on the upper slopes of Dry Creek Canyon, about 3.5 miles from the Smart home.
Her father, Ed Smart, has said Elizabeth could hear searchers calling her name, but didn't respond because she feared for her life.
Richard Townsend, director of the state crime lab, said Friday that his lab has received a "boatload of evidence" from the police investigation.
"We're in the early stages" of assessing evidence "from shoe impressions to weapons to clothing. It continues to funnel in. We're only beginning to start our analysis," he said.
Mitchell and Barzee made their first court appearance Wednesday and were appointed lawyers because they could not afford their own.
Thomas said the family would rather not have her take the stand, "but they're willing to do whatever it takes to ensure that these two individuals receive the proper punishment."