Quadruple murder suspect Bryan Riley is led from the Polk County Sheriff’s Office in Lakeland, Flordia, on Sunday. AP

A former sniper in the US military went on a shooting spree in central Florida, killing four people, including an infant boy in his mother's arms, before surrendering to police in central Florida on Sunday, officials said.

The suspect, identified as 33-year-old Bryan Riley, shot and killed a 40-year-old man, a 33-year-old woman and her three-month-old son in a residential area in Polk County, Florida, said County Sheriff Grady Judd. The fourth victim was the child's 62-year-old grandmother.

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The shooter, identified as a former US Marine, also wounded an 11-year-old girl who underwent surgery for seven gunshot injuries, Polk County Sheriff Grady Judd told a news conference.

Polk County, Florida, Sheriff’s officials work the scene of a multiple fatality shooting on Sunday in Lakeland. AP

A veteran of the US wars in Iraq and Afghanistan, Riley was working as a bodyguard and a security guard. His girlfriend of four years told investigators he suffered from post-traumatic stress disorder and occasional depression, Judd said.

The suspect also told the police, without elaborating, "You know why I did this," according to a statement from Judd published on social media.

Riley was wounded in the gunfight and was rushed to a local hospital, where he again tried to attack police officers and had to be sedated.

After he received medical help, the suspect was transferred to a local jail.

It was not immediately clear what prompted the attack, Judd said. An investigation is underway.