A former American professional football player killed five people including a doctor and the doctor's wife before dying by suicide, police in the US state of South Carolina confirmed Thursday.

Authorities said at a press conference that former San Francisco 49ers cornerback Phillip Adams fatally shot the five victims Wednesday in the city of Rock Hill. 

He then returned to his parents' house, where he killed himself early Thursday after a standoff with police.

"There's nothing right now that makes sense to any of us," said York County sheriff Kevin Tolson, adding that law enforcement have yet to determine a motive.

The five victims include a 70-year-old doctor, his 69-year-old wife and two of their grandchildren, age five and nine, as well as a 38-year-old boiler technician who was killed while working outside their house.

The doctor, Robert Lesslie, was a "pillar" of his community, the sheriff said. He had founded a health care center nearby, according to local media.

"The losses we are suffering cannot be uttered at this time," Lesslie's family said in a statement, read out by Tolson during the press conference.

A sixth person, another technician working at the house, was hospitalized with serious gunshot wounds, Tolson said.

After arriving at the crime scene, the police were quickly able to identify Adams as a suspect based on evidence, according to the sheriff.

After evacuating Adams's parents, who were unaware of what had happened, from their house, authorities sought to reach the former NFL player, who was still inside.

When officers entered the house, they discovered Adams's body, Tolson said. He had died from a self-inflicted wound.

Speaking to a local television channel, Adams's father Alonzo said he believed playing professional football had "messed him up."

Adams's professional career, from 2010-2015, was marred by injuries, including concussions. 

In addition to the 49ers, he also played for the New England Patriots, the New York Jets, the Oakland Raiders and the Atlanta Falcons.

The quintuple murder comes as the United States is still reeling from two mass shootings in early 2021 in Atlanta and Boulder, Colorado.

US President Joe Biden, who unveiled a plan for new gun control measures on Thursday, branded gun violence in the country an "epidemic" and an "international embarrassment."

During his speech, Biden cited the Rock Hill shooting as an example.