Capas said investigators gathered evidence where the worker was found and the places where her vehicle had been parked and later located.
The monument was the site of the still-unsolved 1980 disappearance of a Park Service ranger, Paul Fugate, who was last seen when he went on foot patrol.
Also in the region, the unsolved shooting death of a Douglas-area rancher, Rob Krentz, helped spur passage of a 2010 Arizona law targeting illegal immigration.
Krentz was gunned down in March 2010 while checking water lines on his property. Authorities believe — but have never produced substantive proof — that a scout for drug smugglers killed Krentz.
The U.S. Supreme Court struck down some parts of the law in 2012, including the requirement that immigrants obtain or carry immigration registration papers. But the high court upheld a requirement that Arizona officers question the immigration status of those suspected of being in the country illegally.