The man who killed four people Wednesday at a hospital in Tulsa, Oklahoma, was targeting the doctor who operated on his back and whom he believes was responsible for his pain, police said Thursday.

• Read also: New shooting in the United States: a man kills 4 people and commits suicide in Oklahoma

Michael Louis entered Saint Francis Hospital on Wednesday with a handgun and a semi-automatic rifle, the latest killing in a country still marked by the massacre in a primary school in Uvalde.

In addition to his doctor, the shooter killed another doctor, a receptionist, a patient and injured others before committing suicide, the police said at a press conference.

Michael Louis had back surgery by Dr Preston Phillips on May 19.

After being discharged from the hospital on the 24th, the patient had “called several times complaining of pain, and wanted further treatment,” said Tulsa Police Chief Wendell Franklin.

Michael Louis held his doctor responsible for his pain, and officers found a letter on him "which clearly indicates that he came with the intention of killing Dr. Phillips and anyone in his path," added Ms. Franklin.

On Wednesday afternoon, before going to the hospital, the suspect had purchased an “AR-15 type” semi-automatic rifle, he continued.

This weapon, as powerful as it is controversial, was used at the Robb primary school in Uvalde, where 19 children and two teachers died under the bullets of a teenager on May 24.

Vice President Kamala Harris called on Saturday to ban assault rifles, often used in the massacres that regularly punctuate American news.

These weapons “were designed with a specific purpose: to kill a lot of people in a short time. An assault rifle is a weapon of war that has no place in civil society,” she said.

The recent shootings have reignited the gun debate, with President Joe Biden promising to take action. In Congress, the elected officials negotiate to find a compromise, with a slim hope of success as the divisions are deep.