Oklahoma authorities believe the fatal shooting of four people at a Tulsa medical building was not random, saying the gunman handpicked the facility where he carried out the latest in a series of mass killings to shock the United States.

Police said on Wednesday evening the man entered the building on the campus of St. Francis Hospital with a rifle and a handgun and began shooting.

In addition to the four fatalities, several others were injured before the assailant apparently killed himself. Authorities said they believed the victims were both patients and medical staff. They did not identify them further.

"He very purposefully went to this location, went to a very specific floor, and shot with very specific purpose," Tulsa Police Captain Richard Meulenberg told CNN on Wednesday night. "This was not a random shooting by this individual."

Authorities have said nothing more about the gunman's motive and have not identified him beyond estimating his age at between 35 and 40.

The shooting comes on the heels of two mass killings that have stunned Americans and reopened a long-standing debate over tightening controls on firearms ownership and the role of mental health in the epidemic of gun violence plaguing the country.

About a week earlier, an 18-year-old man entered an elementary school in Uvalde, Texas, and fatally shot 19 children and two teachers. Earlier last month, an avowed white supremacist drove three hours from his home to a supermarket in a Black neighborhood in Buffalo, New York, where he gunned down 13 people, 10 of them fatally.

In Tulsa, a city of 411,000 residents, police arrived at the scene three minutes after receiving a call about a shooting inside the Natalie Building just before 6 p.m. CDT (2200 GMT).

Armed officers ran toward the building immediately, eyewitness video clips show. Police followed the sound of gunfire up to the second floor, and made contact with the victims and the suspect five minutes later, Deputy Police Chief Eric Dalgleish told reporters.

The speed of Tulsa law enforcement's response contrasts with an hour-long delay by officers in Uvalde before they stormed the classroom where the gunman in that shooting had barricaded himself. Their lapse has raised questions about whether police could have saved lives if they acted without hesitation.

Asked by reporters whether police had refreshed their training or thinking about active shooters after the Texas school shooting, Dalgleish said: "I think that's probably fresh on everyone's minds."

"I will say Tulsa revisits that topic regularly. I was very happy with what we know so far regarding the response of our officers," he said.

The White House said President Joe Biden had been briefed on the shooting and offered support to state and local officials in Tulsa.