The alleged gunman in a mass shooting that killed five people in South Carolina — including a doctor and two of his grandchildren — was a former NFL player who had suffered concussions during his sporting career and had been treated by the slain physician.

Phillip Adams, 32, a former cornerback, last played in 2015 for the Atlanta Falcons but he also had stints with the Seahawks, 49ers, Raiders and Jets during a six-year career — which was marred by injuries.

During the 2012 season with Oakland, Adams suffered two concussions during a three-game span, as well a groin injury that ended his season, The Herald reported.

Two years earlier, a broken left ankle cut short his rookie season with the San Francisco 49ers, according to the report.

Adams took his life with a .45-caliber weapon early Thursday, hours after investigators say he shot six people, five fatally, on Wednesday.

The victims included prominent local physician Dr. Robert Lesslie, 70, his wife, Barbara Lesslie, 69, and two of their grandchildren, 9-year-old Adah Lesslie and 5-year-old Noah Lesslie, a source close to the investigation told The Associated Press. They were gunned down in their home.

Adams’ parents live near the Lesslie family home in Rock Hill and the former NFL pro had been previously treated by the doctor, a source told the Associated Press.

A fifth victim, maintenance worker James Lewis, 38, of Gastonia, was fatally shot outside the home, according to the York County Sheriff’s Office.

A sixth person, a second worker at the home who was not immediately identified, was hospitalized with a gunshot wound, sources told The Herald.

Adams, a Rock Hill native, was tracked down by police at his parents’ home — down the street from the gruesome shooting scene, the newspaper reported.

Adams is believed to have killed himself as cops surrounded the residence, a coroner told the newspaper.

The former NFL player’s mother was inside the home as cops closed in, but she was safely removed before Adams was found dead, according to the report.

A motive in the mass shooting has not been released by authorities.

Adams’ father, meanwhile, reportedly insisted his son was a “good kid” who was “messed up” from his football playing days.

Alonzo Adams told WCNC he was unaware of his son being violent prior to the massacre.

“I can say he was a good kid,” Alonzo Adams said. “I think the football messed him up. I don’t think he ever did anybody any harm.”

Adams was convicted of several traffic charges in York County on Tuesday, one day before the mass shooting, including driving under suspension, The State reported, citing South Carolina records.

He was also charged with simple assault and battery in 2009, but was later acquitted at trial, the newspaper reported.

Adams was drafted in the seventh round by San Francisco in 2010 after attending South Carolina State University. He played 78 games in his six-year career, according to a pro football reference website.

NFL officials acknowledged Wednesday’s mass shooting in a statement to The Post Thursday.

“Our hearts go out to the families and friends of the victims of this devastating tragedy,” the statement read.