PEOPLE learns that a North Carolina man already charged with his son’s 1998 murder will likely face additional criminal charges sometime next month.

Orange County Assistant District Attorney Anna Orr would not discuss the case in specifics, but says more charges are coming for John Russell Whitt, who appeared in court on Monday after a grand jury indicted him in May.

Whitt, 57, is accused of killing his 10-year-old son, Bobby Whitt, and leaving his body beneath a billboard along Interstate 85 in Mebane, North Carolina.

Detectives suspect Whitt also killed his wife, Myoung Hwa Cho, 44, Orr confirms — noting that next month’s added charges will be related her to death.

Police believe Cho was suffocated in May 1998; her naked body was found in Spartanburg, South Carolina, which is also along Interstate 85.

Cho was suffocated, medical examiners determined, while Bobby died from strangulation — most likely in July 1998. His skeleton was found beneath a billboard by work crews cutting grass along the highway.

Whitt, who remains in custody without bond, has yet to enter pleas to first-degree murder and concealment of death stemming from his son’s killing. A second first-degree murder charge is being mulled over by prosecutors.

PEOPLE was unable to reach his attorney for comment.

For years, Bobby’s identity was unknown, as he was never reported missing. Relatives allegedly believed Cho returned to her native South Korea, taking the boy with her.

Bobby was identified after his DNA results were run through genealogy websites.

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Whitt has been behind bars since 1999, serving a sentence in federal prison after pleading guilty to six counts of armed bank larceny.

He won’t be released on those charged until 2037.

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  By Chris Harris