The gunman who killed 4 people inside a Tulsa hospital bought his AR-15 style rifle just hours before the mass shooting: police

• The gunman who killed four people at a Tulsa hospital bought a rifle just hours before the shooting.
• Authorities said an AR-15 style rifle was purchased shortly before Wednesday's mass shooting.

The gunman who killed four people at a Tusla hospital on Wednesday bought an AR-15 style rifle just hours before the mass shooting, authorities said.

Tulsa Police Chief Wendell Franklin said at a Thursday press conference that the gunman purchased a semi-automatic rifle from a local gun store at 2 p.m. on June 1, three days after buying a semi-automatic handgun at a pawn shop.

Just a few hours later at 4:52 p.m., a series of 911 calls from the hospital began to stream in about an active shooter in a medical building at St. Francis Hospital, Franklin said.

The gunman had come to the hospital with the intent to kill a doctor whom he blamed for pain after a recent back surgery, Franklin said.

A letter found at the scene leads investigators to think the gunman was prepared to shoot "anyone who got in his way," Franklin added.

Franklin said the doctor is among the dead, including the gunman who apparently died by suicide.

Authorities said investigators found 30 bullet casings from a semi-automatic rifle and seven from a handgun at the scene.