Oliver Pareece Jones, 37, stopped at the Walmart on Foothill Boulevard at about 8:45 a.m. April 5 on his way to Las Vegas. He has not been heard from since.
Jones was visiting Southern California when he was attacked at a nightclub in Los Angeles, his family said. He ended up at Cedars-Sinai Hospital with a concussion.
"We're thinking that his concussion really affected his ability to reason or be able to do anything," Jones' brother Jared Jones said.
Jones checked himself out of the hospital, then stopped at the Walmart in Rancho Cucamonga on his way out of town, family members said.
Private investigator Paul Huebl suspected foul play.
"He bought some stuff and apparently a transient wound up with some of the stuff," Huebl said. "It doesn't make sense, because it's kind of expensive stuff: a cell phone, a radio, some other things."
Jones' family said he normally called his daughters every day, but they haven't heard from him.
"If there's a little bit of hope, we're going to hold onto that," Jared Jones said.
Jones was described as having blond hair and blue eyes, 5 feet and 8 inches tall and 170 pounds.
Anyone with information is asked to call the Rancho Cucamonga Police Station at (909) 477-2800.