Timeline: How the Tulsa medical office mass shooting unfolded

The suspected gunman in a mass shooting at an Oklahoma medical office Wednesday bought an AR-15-style rifle just hours before the massacre, police said.

Five people were killed in the shooting at a medical complex at Saint Francis Hospital in Tulsa, according to the Tulsa Police Department. The alleged shooter died of a self-inflicted gunshot wound, police said.

Among those killed was Dr. Preston Phillips, who had performed back surgery on the alleged gunman last month, Tulsa Police Chief Wendell Franklin said at a news conference Thursday.

A letter recovered on the suspect -- identified by police as Michael Louis, 45, of Muskogee, Oklahoma -- "made it clear that he came with the intent to kill Dr. Phillips and anyone who got in his way," Franklin said. "He blamed Dr. Phillips for the ongoing pain following the surgery."

The three others killed were Dr. Stephanie Husen, receptionist Amanda Glenn and patient William Love, Franklin said.

As police continue to investigate the incident -- the 233rd mass shooting in the U.S. so far this year, according to the Gun Violence Archive -- this is what is known so far.

"This information is fluid and can still change but it is the most accurate information that we have at this point," Franklin said.

All times Central.

May 19

Louis goes into the hospital for back surgery performed by Dr. Phillips.

May 24

Louis is released from the hospital. After being released, he "called several times over several days complaining of pain and wanted additional treatment," Franklin said.

May 29

The suspect purchases a semi-automatic handgun, a 40-caliber pistol, from a local pawn shop, according to Franklin.

May 31

Phillips sees Louis again for additional treatment.

June 1

The suspect "calls again complaining of back pain and wanting additional assistance," Franklin said.

2 p.m.: The suspect purchases an AR-15-style rifle from a local gun store, according to Franklin.

4:52 p.m.: A patient who was on a video chat with a doctor at the medical office calls 911, saying "the doctor told her to call 911 saying there has been a shooting," Franklin said.

4:53 p.m.: The Tulsa Police Department receives a 911 call reporting a shooter in a building.

4:55 p.m.: A 911 caller identifies the shooting location as the Natalie Building, a five-story medical office building on the hospital's campus. Other calls report a shooting on the second floor of the building.

4:56 p.m.: Police arrive on the scene.

4:58 p.m.: As police advance toward the suspect's location, they hear a gunshot, which is believed to have been the suspect taking his own life, Franklin said.

Police conduct a search of the building and find a victim in an exam room, Love, who later died at the hospital, Franklin said. He was shot while holding a door closed to allow someone else to escape out of another door, according to Franklin.

A woman was found unharmed hiding under a desk in the lobby area of the office at the suspect's foot and witnessed the suspect take his life, Franklin said. An additional victim is found dead next to the suspect, he said.

Phillips is found dead in an exam room and another victim is found dead in an open area near a nurse's station, Franklin said.

5:24 p.m.: The Cherokee County Sheriff's Office calls Tulsa police to report receiving a call from a woman "saying that her husband had killed multiple people at Dr. Phillip's office," Franklin said. The suspect appears to have contacted his wife either before or during the shooting, he said.

A letter was found on the suspect that "told us the story," Franklin said. "This was something that was planned."

6:33 p.m.: The Muskogee Police Department reports a possible bomb threat inside a residence in Muskogee, about 50 miles southeast of Tulsa. Muskogee Police Chief Johnny Teehee later updates that the incident may be tied to the Tulsa shooting suspect and that officers have cleared the house and are obtaining a warrant to search the residence.

9:06 p.m.: Muskogee Mayor Marlon Coleman updates on Facebook that the "bomb squad has cleared the location of the potential bomb threat."

The home is believed to be the suspect's residence, but police are still working to determine what happened, Franklin said Thursday.

ABC News' Jenna Harrison Esseling contributed to this report.