Chicago police map areas where they may dig

CHICAGO (CNN) -- Police may begin digging Monday at a site where four more victims of executed serial killer John Wayne Gacy may be buried.

Authorities plan to use ground sonar on Monday and, depending on the results, could begin to excavate the site immediately afterward.

The location on Chicago's Northwest side, a parking lot outside a brick apartment building where Gacy's mother once lived, is four miles from Gacy's suburban home where authorities discovered most of his 33 victims in 1978.

For six years starting in 1972, Gacy lured young men and boys to his home for sex, then tortured and strangled them. Most of Gacy's victims were found in the crawl space beneath his home. Gacy, a part-time clown and building contractor, was put to death in 1994.

A private investigative group, the Better Government Association, first conducted a ground sonar on the new site after talking with a retired Chicago police detective who recalled seeing Gacy there with a dirty shovel one night in 1975.

That ground-penetrating radar probe produced images of what appeared to be skeletal remains of at least four bodies. One image looks like a skull, ribcage and shoe.

Police believe there may be more victims than the 33 previously discovered

Authorities plan another use of the ground sonar on Monday and, depending on the results, could begin to excavate the site.

The man whose recollection led to the new developments is former detective Bill Dorsch. He told the Chicago Tribune, "I thought back to the time I'd found him in the middle of the night, holding the shovel, and asked myself, 'Was he burying a body back then?'"

After police arrested Gacy in 1978, Dorsch said he told investigators about his encounter with Gacy but that nothing came of the tip.

Dorsch said Gacy and he were acquaintances at the time and that when he asked Gacy what he was doing there so late with a shovel, Gacy smiled and said he was doing some unfinished digging related to his work.

Chicago Bureau Chief Jeff Flock and Reuters contributed to this report.