By CBS3 Staff

PHILADELPHIA (CBS) — A suspected serial killer who authorities say shot and killed a Dunkin’ manager during an armed robbery earlier this month has been charged with murdering his own mother. Philadelphia District Attorney Larry Krasner approved murder charges Wednesday against Keith Gibson in connection to the February fatal shooting of his mother, Christine Gibson.

Christine Gibson was at work in East Falls, Philadephia when she was killed on Feb. 8. Loved ones feared her only son had something to do with it. Her body was discovered by her colleague and friend, Linda Osuigwe.

READ MORE: Southwest Philadelphia Block Renamed After Rev. Paul 'Earthquake' Moore

“We were suspicious, but we had no proof,” Osuigwe said earlier this month.

Gibson is suspected of a series of homicides, starting with his mother’s. He was arrested in Wilmington, Delaware, after an armed robbery at the West 4th Street Rite Aid on June 8 just days after authorities say he shot and killed 41-year-old Christine Lugo during a robbery at a Dunkin’ in North Philadelphia.

READ MORE: Commuters Make Backup Plans Ahead Of SEPTA Strike Vote

Police say Lugo was opening the Dunkin’ located on the 500 block of West Lehigh Avenue in Philadelphia’s Fairhill section just before 5:30 a.m. on June 5 when Gibson fatally shot her during a robbery. According to police, after the Dunkin’ shooting, he shot and critically wounded a 19-year-old in Wilmington and pistol-whipped a female worker at a Rite Aid in New Castle.

On May 15, officials say he killed a worker at a T-Mobile store in Elsmere, Delaware.

Gibson, in 2008, was convicted and served time for committing a homicide and commercial robbery in Delaware.

MORE NEWS: Camden County Voters Trickle In As New Jersey Starts Early Voting For The First Time

The Philadelphia District Attorney’s Office says Gibson remains in custody in Delaware.