Arkansas prosecutors charged two more teenagers late Thursday in connection with the shooting deaths of one of their friend's grandparents.

Faulkner County prosecutors filed adult first-degree murder charges against Conner Atchley and Anastasia Roberts, who are both 17. Investigators allege that the teens were both present when Robert and Patricia Cogdell were fatally shot in their Conway home in July.

Prosecutors have already charged 14-year-old Justin Staton, whom the couple raised as their grandson, and his friend, 18-year-old Hunter Drexler, with capital murder. Investigators allege they robbed and shot the couple, then dumped their bodies in a wooded area behind their home.

Prosecutors also filed hindering apprehension charges against Staton's mother, Michelle Staton, on Thursday.

Prosecuting Attorney Cody Hiland said the investigation is ongoing. He said he could not discuss the facts of the case and referred reporters to the affidavits filed late Thursday in Faulkner County Circuit Court.

He said Atchley, Roberts and Michelle Staton were in police custody and would be served the paperwork from the charges.

No attorneys were listed for Atchley, Roberts or Michelle Staton. A phone call to the Arkansas Public Defender Commission was not returned late Thursday. Attorneys from the commission are representing Justin Staton and Drexler.

Court documents say Justin Staton told officers that he complained to Drexler about his grandparents and that his friend suggested that he shoot them. Atchley, who was present during the attack, told officers the plan had been conceived while he, Justin Staton and Drexler were in juvenile detention together, according to the documents.

The Cogdells were not Staton's biological grandparents, but they had raised him as if they were, court documents say.

Justin Staton and Drexler have pleaded not guilty to capital murder, aggravated robbery and other charges.

The affidavits filed Thursday describe how the teens said in interviews that Drexler took the weapons used in the shooting from his dad's collection and that Atchley was involved because of his "vast knowledge of marijuana (buying, selling and growing)." Atchley told Roberts about the plan to kill the Cogdells after he was released from detention, according to the affidavits.

The teens told police they had a plan to run away together and that Staton had promised them a BMW for their help, the affidavits say. Staton paid Atchley $1,000 and Drexler $500 from cash taken after the shootings, according to the affidavits.

Police found the Cogdells' bodies after a co-worker of Robert Cogdell's from the city of Maumelle's Public Works Department became concerned that he had not come to work.

Atchley, Roberts and Drexler fled Arkansas and were later apprehended in Graham, Texas. The teens were sought as runaways and were brought back to Arkansas for questioning more than a month ago.