The gunman who stormed a Tulsa, Oklahoma medical center on Wednesday, June 1, was searching for a "Dr. Phillips," according to witnesses.
The shooting occurred in an orthopedic clinic on the second floor of the Natalie Building at St Francis Hospital.
Four people were killed, before the gunman turned the gun on himself.
The shooter was armed with a handgun and an assault rifle that was purchased earlier that day, police say.
Officers responded to the scene within minutes and immediately ran toward the sound of the gunfire on the second floor.
"The officers that did arrive were hearing shots in the building, and that's what directed them to the second floor," Deputy Chief Eric Dalgleish said at a press conference on Wednesday.
Investigators are still working to identify the shooter. They said the shooting was not random.
"This wasn't an individual who just decided he wanted to go find a hospital full of random people. He deliberately made a choice to come here and his actions were deliberate," Dalgleish said.
Authorities served a search warrant for explosives at a residence in Muskogee, but no explosives were found in the house.
The mass shooting comes eight days after an 18-year-old armed with an assault rifle killed 19 children and 2 teachers at Robb Elementary School in Uvalde, Texas.
The Uvalde Police Department was heavily criticized for waiting in a hallway for over an hour while children bled to death in a classroom.
The police department has stopped cooperating with a federal investigation into their cowardly actions.
The school will be torn down and rebuilt in a different location after children expressed fears of returning to the building where their schoolmates died.