Harrison also says the body will be sent to the medical examiner's office for a positive identification. Results are expected on Tuesday morning.
Cary police are investigating whether the body belongs to Cooper. Wake County Sheriff's deputies are working with Cary Police as they investigate at the scene.
However, authorities say they have notified Cooper's family in person to let them know about the unidentified body.
"We're not making any assumptions that this is Nancy, but we are working with the Wake County Sheriff's Office to identify the body," said Cary Police Chief Pat Bazemore. "It is a sad and difficult time for all of us. Our hearts are with Mrs. Cooper's family and close friends during this very trying time."
Eyewitness News will have more information as it becomes available.
Meanwhile Monday, police and volunteers widened their search for Nancy Cooper. Some looked deep in the woods, and others retraced her steps on paths where she's jogged before.
"So many people started to come out, so many people started to help, that instead of random searches they had a whole lot of people working together," searcher Nick Vasquez said.
Jogger Andrea King is also participating in the search. "Hope that maybe she had an accident and just fell down, and you know, just keep beating the pavement until we find her!"
Officers say Nancy Cooper, 34, left her home at 7 a.m. Saturday to go running and never came back home. Search teams have been searching Lockmere Lake Park and trails she frequently ran in the Regency Park area.
Police say Cooper's husband was the last person to see her Saturday morning, but do not consider him a suspect.
"We've been in constant communication with the husband since Saturday. And we're talking directly with him," Cary Police Chief Pat Bazemore said. "We have not named a person of interest or a suspect."
Bazemore says Cooper's husband did not report her missing. He did not say why Brad Cooper didn't report his wife missing.
"She went out running, was going to help friend paint and didn't show up. A friend called 911 just before 3 p.m. to report her missing," Bazemore said. "The last person who actually saw her was Brad Cooper when she came home that night and first thing in the morning."
No word of the kids also saw her. The children have been with father, family and friends. Cooper's parents Garry and Donna Rentz, along with her identical twin sister Khrista Lister, arrived in the Triangle Monday from Canada.
"It's extremely important that family members are here and helping us," Bazemore said.
Nancy's husband Brad said a few words at a late afternoon press conference Monday. He thanked volunteers and everyone for helping out.
Friends say Cooper would not simply pick up and leave her family and two young daughters, who are ages 4 and 2.
"We've had a lot of call-in's from citizens giving information to us. We're running down those leads, ruling each one of those out as we get to them," Cary Police Department Capt. Michael Williams said.
Michael Morwick's wife is Cooper's best friend. He says they'll search for her as long as it takes. "We know that Nancy's a strong girl, and we know that she is out somewhere. So we're going to keep looking until we find her," Morwick said.
Morwick tells Eyewitness News that Nancy Cooper's husband and two young daughters are waiting for family from Canada to arrive.
"They're very concerned. They will be coming down shortly. We don't exactly know when, they're coming from Canada. But they're coming, and hopefully will find an end to this."
Cooper is described as 5 feet 9 inches tall, 120 pounds, hazel eyes and brown hair. She was last seen wearing a T-shirt, running shorts and light blue running shoes.
Cooper did not have any keys, phone, or ID on her when she left.