A gunman who killed four people at a hospital in Oklahoma targeted the doctor who performed back surgery on him, the Tulsa Police chief said.

The man who carried out the shooting at the St Francis Hospital campus in Tulsa on Wednesday was identified as Michael Lewis by police.

The gunman blamed the doctor – Dr Preston Phillips – who carried out his back surgery for the pain he was in. Dr Phillips was one of those killed in the attack.

Another doctor and two other people were killed after the gunman opened fire with an AR-15-style weapon – the same type of semi-automatic gun used by the shooter in the recent Texas school shooting.

He had purchased the gun on the same day he carried out the shooting, the police chief added.

Lewis killed himself after shooting the four people in a spree that lasted about five minutes.