Investigators were led to Mahmud and Christopher Parks, 23, and Ashanier White, 19, both of Philadelphia, after poring over their texts and posts on Facebook.
Parks, Whelan said, drove the getaway car and may have provided the murder weapon for the two people already in custody, charged with entering the store on Sept. 19 and killing McClay. Rita Pultro and David Wiggins face first-degree murder charges. Mahmud, Parks, and White were charged with second-degree murder, conspiracy, and related offenses.
Whelan said that investigators recovered deleted text messages from Pultro's and Wiggins' cellphones and searched their Facebook pages to track down their accomplices. "We were able to piece this together like a jigsaw puzzle," he said.
Although between the robbery and their arrest Pultro and Wiggins had discarded their cellphones and obtained new ones, he said, investigators were still able to read text messages linked to their accounts. They used Facebook photographs and comments as well as anonymous tipsters' guidance to link the people mentioned in the text messages - identified only by nicknames - to the three additional suspects.
"Smartphones give us a tremendous amount of information," Whelan said.
White helped make sure that Parks would be available to drive the car and that Pultro and Wiggins would have a weapon available to them, Whelan said.
Reading the text messages, police learned that the conspirators had planned to rob the store two nights earlier. That plan was aborted because the suspects did not have the money to buy gas.
Whelan said the safe contained about $5,000, the amount Mahmud predicted in his texts.
Police also learned that Pultro, a bartender at a place frequented by Wiggins, joined the conspiracy at the last minute. The others had been planning the robbery for at least a week.
Whelan said Pultro and Wiggins have each accused the other of firing the shot that killed McClay, 40, of Broomall. Investigators believe Pultro pulled the trigger.
According to a police affidavit, Pultro was previously charged with repeatedly slashing a woman she knew with a razor, causing the victim to receive 46 stitches. She is due back in court on those charges Oct. 16.
Mahmud was fired by Rite Aid within the last two weeks, Whelan said, for causes unrelated to the robbery.
He said police are not seeking any other suspects: "We believe this completes the circle of individuals involved."
Inquirer staff writer Robert Moran contributed to this article.