KANSAS CITY, Mo. -- The family of a woman who vanished more than a month ago has hired a private investigator to help in the search for her.

Summer A. Shipp, 54, disappeared Dec. 8 while conducting door-to-door market research in an Independence neighborhood. Police later found her car, but no sign of her.

"I'm convinced she's being held somewhere. I'm not sure who took her or why," daughter Brandy Shipp said.

Despite a lot of publicity and repeated search efforts, investigators have not been able to turn up anything. Officers searched a home in the neighborhood where Shipp disappeared and arrested a man on unrelated warrants. He was held for several weeks, but was released on bond Wednesday night.

"Keeping her picture and her name out there in the public eye is going to lead to someone who knows something," Brandy Shipp said.

A benefit movie screening will be held Thursday evening to raise money so the family can continue to pay for the fliers with Shipp's picture and to help pay to hire a private investigator.

"We're going to find her. People don't just vanish into thin air -- they don't. That doesn't happen. We're going to find her," Brandy Shipp said.