(CBS) —  Outside Detroit, investigators continue to search a wooded area where a suspected serial killer is believed to have buried his victims. Authorities say the suspect — serving a life sentence for the murder of a 13-year-old — recently bragged to fellow inmates about killing as many as seven girls decades ago.

A tireless search on the ground and from above have authorities confident they are close to solving a 40-year-old cold case.

“We do have probable cause to believe that this is a gravesite,” said Warren Police Commissioner Bill Dwyer.

Police say they started searching here after convicted killer Arthur Ream boasted to fellow inmates that he killed several girls. They also say he failed a recent polygraph test related to the crimes.

One of the missing girls, Kimberly King, disappeared in 1979 when she was 12 years old. Her best friend, Annie Dunaca, was the last person to see her alive.

“It would be nice to start the healing process rather instead of the anxiety of the ‘Where is she? Is she ok?'” Dunaca said.

In 2008, Ream led investigators to the remains of another missing girl buried in the area, 13-year-old Cindy Zarzycki. In an undercover video from Detroit station WDIV, Ream described where and how he buried Cindy’s body.

“All she had to do was go up North Avenue,” Ream said. “And it’s right there.”

“Now did you go pretty deep, or — ” the investigator asked.

“About four feet,” Ream answered.

Authorities say they have little doubt Ream is responsible for the murders of up to seven girls. According to one court document, Ream “admitted to having a fetish for 13- and 14-year-old girls.”

On Wednesday, Cindy’s father, Ed Zarzycki, returned to the site where his daughter was found 10 years ago.

“I don’t know if there’s ever justice,” Zarzycki said.

Kimberly King’s family say they believe a podcast about the missing girls that ran in the fall prompted authorities to re-open the cold case. Investigators say they expect the search to take another two to five days.