McClay was closing the store when the two walked in, with the intent to rob the store, police said.
Customers and employees heard a loud bang; officers found McClay shot in the head, and he was pronounced dead at the scene.
McClay was a Navy veteran and a graduate of Haverford High School.
Wiggins was on probation at the time of the shooting, according to court records.
Using fingerprints left at the scene, investigators identified Wiggins as a suspect within four hours of the shooting, Chester Police Commissioner Joseph Bail said.
Police were mounting an intensive search Saturday night for Wiggins' alleged accomplice, and they have the identity of a suspect. The accomplice was last seen wearing a red hoodie with white stripes down the sleeves, blue jeans, and gray sneakers.
"I'll be much happier when we have the female in custody," Bail said. He said the woman does not live in Chester.
Police, who had released surveillance-tape images to the public on Friday, received tips that led them to the woman's identity, he said.
Delaware County District Attorney Jack Whelan could not be reached for comment last night.
Wiggins has a history of minor brushes with the law, court records show.
He was arrested in February 2010 by Philadelphia police for a December 2009 car theft. He pleaded guilty, and in October 2010 was sentenced to up to 23 months in jail and three years' probation.
He was released in March 2011 and ordered to undergo random drug screens and drug treatment. He was also ordered to get a job.
Five months later, while on probation, Wiggins was arrested for attempted burglary and related charges.
He pleaded guilty and was sentenced to four years' probation and ordered to undergo drug and alcohol screening and treatment.
In March, he was picked up for a probation violation.
A judge ordered Wiggins back into drug treatment and said he had to take "life skills" classes at the University of Pennsylvania.
A $20,000 reward had been announced for information leading to an arrest in the killing, funded by Rite Aid and Purdue Pharma L.P.
Chester police asked anyone with information to contact Chester Police Detective Nelson Collins at 610-447-8431 or Delaware County CID Detective Dave Tyler at 610-891-4708.
Contact Jonathan Lai at 856-779-3220, email@example.com, or on Twitter @elaijuh.
Staff writer Mike Newall contributed to this article.