Peoria, Ill. ? Police on all-terrain vehicles, horseback and even airplanes have combed fields and forest in search of one or more serial killers believed responsible for the deaths of six women whose bodies were dumped along little-traveled Illinois roads.

The women, all of them black with a history of prostitution or drug abuse, had cocaine in their systems when they were strangled or died of drug overdoses over the past 3 1/2 years. None showed signs of a struggle, investigators said.

Until a suspect is identified, authorities are trying to reassure nearly 312,000 residents of two largely rural counties southwest of Chicago, while also cautioning them to be on guard.

“I don’t think anybody’s entirely safe,” Peoria Mayor Dave Ransburg said. “If someone can do what they’ve done to these six women, they could do it to someone else.”

Deputies in Peoria and Tazewell counties joined city and state police in September to form a 13-person task force devoted solely to the unsolved cases that have mounted since March 2001, when the first body was found near tiny Pottstown. Four other women have been reported missing.

“If you take a surface look at everything, it’s easy to say, ‘Absolutely, it’s a serial killer,”‘ Peoria County Sheriff Mike McCoy said. “But when we get into it a little bit more, I tend to think there might be more than one person.”

The women were identified as Sabrina Payne, Frederickia Brown, Brenda Kay Erving, Linda Neal, Wanda Jackson and Barbara Williams.