CLEVELAND (AP) – The FBI said Friday it has joined an investigation by police and county prosecutors to determine if there are unsolved slayings that might be attributed to a Cleveland trucker driver now linked to five slayings over the last 18 years.

Robert Rembert Jr., 45, pleaded not guilty and was ordered held on a $5 million bond during a court appearance in Cleveland on Friday. Rembert is charged with multiple counts of aggravated murder in four slayings – three this year and one in 1997. He already served prison time for a 1997 shooting death.

Cuyahoga County Prosecutor Tim McGinty has called Rembert a serial killer and has said investigators will examine truck routes Rembert drove in Ohio and Pennsylvania to see if he can be tied to any unsolved slayings. An FBI spokeswoman on Friday confirmed that agents would be assisting prosecutors and Cleveland homicide detectives in the investigation.

One of Rembert’s newly assigned attorneys declined to comment on Friday.

Prosecutors have said DNA evidence ties Rembert to the strangulation deaths of 47-year-old Rena Mae Payne in 1997 and 31-year-old Kimberly Hall this June. Prosecutors say the two women were sexually assaulted. Rembert is also charged with fatally shooting 26-year-old Morgan Nietzel and his cousin, 52-year-old Jerry Rembert, last month at a Cleveland home he shared with the couple.

In 1998, Rembert pleaded guilty to voluntary manslaughter and was sentenced to six years in prison for the fatal shooting of 24-year-old Dadren Lewis.

Sheriff’s deputies arrested Rembert as he emerged from a shower at a truck stop outside Cleveland on Sept. 21, the day after the bodies of Nietzel and Jerry Rembert were discovered. Both had been shot in the head. Nietzel’s car was found at the Medina County truck stop.

Payne’s body was discovered in an employee restroom at a Regional Transit Authority bus turnaround. Rembert was an RTA bus driver at the time and knew the entry code for the restroom, prosecutors said.

Hall’s body was found in a field on Cleveland’s east side.

Besides 10 counts of aggravated murder, Rembert also was indicted on charges of kidnapping, rape, aggravated robbery, grand theft and gross abuse of a corpse.

“Robert Rembert is a serial killer,” McGinty said earlier this week. “So far, we know he’s purposefully executed five people.”