That's the question police and Nancy's friends and family are trying to answer.
Her husband Brad explained what happened in an affidavit filed in the custody battle over the couple two young daughters.
In one part of the 13-page document he states, "Nancy left for her run around 7 a.m.. She usually ran with her friend, Carey, who lives in a condo a few minutes from our house."
But in a new affidavit submitted this week in the custody case, Carey Clark said, "I have no plans to run with /*Nancy Cooper*/ on Saturday, July 12, 2008 and I never ran with Nancy Cooper on July 12, 2008."
That statement does not refute Brad's statements since he has never said he was certain that Nancy went running with Clark.
He simply stated that when none of Nancy's friends knew where she was he thought she might have run with Clark.
The latest affidavit from Clark is apparently not a revelation to investigators and Nancy's friends.
"We knew that Nancy did not run with Carey that morning," Nancy's friend, Susan Crook, said.
Crook has been asked by some of Nancy's closest friends to speak on their behalf.
She says the day Nancy was reported missing, she was with a group of Nancy's friends and actually met Clark for the first time.
"We saw Carey and asked her if she had run with Nancy that morning," Crook said. "She said that she had not nor did she have plans to run with Nancy that morning. So it's not a surprise. The only surprise is that it's taken this long to come out."
But it has only taken this long for the information to become public. Investigators had the information early in the case.
It doesn't necessarily help them because in Brad's sworn statement, he is uncertain about who Nancy ran with.
He told investigators several hours after Nancy went running, "I began calling Nancy's friends to see if Nancy was with them and to ask if anyone had Carey's phone number to see if Nancy went running with her."
But Nancy's friends think it's all a lie.
"There's great speculation that Nancy did not go running Saturday morning," Crook said.
Some have done more than speculate.
In their affidavit, Nancy's parents and her twin sister say they don't believe she went running that morning. Another friend said she believes Brad killed Nancy.
Brad has not been charged, arrested or named a suspect or a person of interest.
Nancy's body was found by a man walking his dog on the evening of July 14. Her body was located in the Oaks at Meadow Ridge neighborhood, which is under construction. It is in a 5-mile radius of the Cooper home.
On July 25, a judge granted Nancy's parents custody of Brad and Nancy's two daughters. The children will live in Canada, but will have supervised visits with their father.