The gunman who shot and killed four people at a hospital in Tulsa, Oklahoma, before taking his own life had been a back surgery patient

According to authorities, the shooter who shot and murdered four people at a hospital in Tulsa, Oklahoma, before killing himself, was a back surgery patient who had lawfully purchased the assault weapon he used in the shooting less than three hours before the tragedy.

Michael Louis, 45, was found with a letter stating his intention to kill Dr. Preston Phillips, a Harvard grad who travels the world providing surgeries to underprivileged children and operated on the gunman on May 19, as well as ‘anyone who got in his way,’ according to Tulsa Police Chief Wendell Franklin.

Franklin said Louis had been released from the hospital on May 24 but called Phillips about lingering back pain and saw the doctor again on May 31, before returning the following day to kill the surgeon, along with Dr. Stephanie Husen, receptionist Amanda Glenn and patient William Love, a retired US Army veteran who was shot while holding a door open so others could escape.

Franklin added that Louis had used an AR-15 style semiautomatic rifle that he purchased at a local gun shop less than three hours before the shooting and was also armed with a .40 caliber handgun, which he purchased on May 29 at a pawn shop.

Police have recovered a total of 37 shell casings at the hospital, 30 from the rifle and seven from the pistol.

The deadly shooting took place eight days after an 18-year-old gunman in Uvalde, Texas, shot and killed 19 students and two teachers at the Robb Elementary School.

Michael Louis, 45, killed himself and four others on WednesdayDr. Stephanie Husen was among the three other victims who were shot and killed in the rampageReceptionist Amanda GlennA woman cries while talking on the phone outside at Memorial High School where people were evacuated from the sceneFamily members hug as they are reunited at Memorial High School after being evacuated from the scene of a shooting at the Natalie Medical BuildingCaptain Richard Meulenberg of the Tulsa Police Department told reporters: 'It's a catastrophic scene' when describing the chaos police encountered when they first entered the hospitalThe Tulsa Police Department seen outside a St. Francis medical building in Tulsa responding to the shooting on WednesdayPolice tape outside of the Natalie Medical Building in Tulsa as police respond to a shooting that killed five people

President Biden was briefed on the shooting and said the White House was monitoring the situation.

‘The White House is closely monitoring the situation and has reached out to state and local officials to offer support,’ officials said in a statement.

Oklahoma Republican Governor Kevin Stit also released a statement condemning the ‘senseless’ shooting and said he is offering Mayor G.T. Bynum any state resources that he may need.

‘What happened today in Tulsa is a senseless act of violence and hatred. Sarah and I are praying for the families of those who lost their lives and for those who were injured. I am grateful for the quick and brave actions of the Tulsa Police Department and other first responders who did their best to contain a terrible situation,’ he said.

‘I have offered Mayor G.T. Bynum any state resources that may be needed, and I ask all Oklahomans to come together in support of the St. Francis Health System community and to grieve with those whose lives have been forever changed,’ Stit added.

The Tulsa Police Department arrives at the St. Francis medical building following reports of the shootingPictured: A map showing the states that saw the highest number of gun violence deaths over the weekend, that saw more than 150 people killed in total across the United States

The mass shooting comes just eight days after an 18-year-old gunman opened fire at Robb Elementary School in Uvalde, Texas, killing 19 students and two teachers before authorities shot and killed him

The tragic shooting has reignited the conversation around gun control.

House Democrats are already planning a vote next week on ‘red flag’ legislation, which allows a judge to ban dangerous individuals from having a firearm, and the Judiciary committee is holding an emergency session on Thursday to mark up right gun-violence bills, which includes measures to raise the minimum age for buying certain weapons to 21.

While Republicans see improving security at schools and tackling mental health problems as a more pressing need than restricting the availability of guns.