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The Oklahoman via AP, Steve Gooch
Oklahoma City police officers gather information from vehicles leaving Will Rogers World Airport, Tuesday Nov. 15, 2016, in Oklahoma City. The airport was put on lockdown after a shooting in the main terminal.

OKLAHOMA CITY — Oklahoma City police are investigating why a man waited to gun down an airline employee outside Will Rogers World Airport in a deadly ambush that forced hundreds of travelers to take shelter and prompted authorities to shut down the state's busiest airport for hours.

Later Tuesday, police discovered the suspected shooter dead from an apparently self-inflicted gunshot wound inside a pickup truck in a parking garage near where they believe he waited, watching airport employees come and go before shooting and killing Michael Winchester, 52, a Southwest Airlines employee.

"We do believe this was a premeditated act against the victim," said Oklahoma City Police Capt. Paco Balderrama. "This was not random, and obviously the investigation is ongoing."

Normal operations resumed Wednesday morning at the airport after 25 flights were canceled Tuesday because of the shooting.

Balderrama said late Tuesday that investigators hadn't positively identified the shooter, but said the man apparently knew Winchester's schedule and routine and that the two men likely knew one another.

"It's too early in the investigation to figure out the exact motive," Balderrama said.

The 1 p.m. shooting set off a scramble at the airport, with police immediately closing the sprawling complex and asking passengers inside to seek cover.

They diverted incoming flights and refused to give already-loaded aircraft permission to leave. There were concerns the gunman might have entered the terminal and mingled among passengers or employees.

"We have a heightened level of security all the time. These people have access to aircraft so we're very concerned about that," airport spokeswoman Karen Carney said.

Winchester — whose address is listed as Washington, Oklahoma, about 35 miles south of the airport — was a former University of Oklahoma football player whose son James is a player for the Kansas City Chiefs. The NFL team tweeted Tuesday night: "Our love and support is with James and the entire Winchester family."

Southwest Airlines canceled flights out of the city for the remainder of the day even though the rest of the terminal reopened.

Hundreds of people were stranded inside the terminal for more than three hours before officers began letting them leave slowly. Carney said about 300 people were held on aircraft away from the terminal after their planes landed ahead of a ground stop.

Police found the suspect's red pickup truck on the second floor of a public parking garage about three hours after the shooting and used a robot to determine the suspect was dead, then gave an all-clear.

The airport handles between 7,000 and 8,000 passengers daily for Alaska, Delta, Southwest and United airlines and has a separate terminal that serves as a transfer center for federal inmates. A jet carrying inmates to the transfer site was allowed to land while the rest of the airport was shuttered.

Video from a television station helicopter showed what appeared to be a pool of blood about 100 feet from the airport's employee parking area — and about 100 yards from the airport's ticket counters and departure area.

While airports have high security, Balderrama said he didn't know whether surveillance cameras might show the shooting.

Balderrama initially said police had received reports of a possible second victim, but no one was found.

Michael Winchester was a punter on the University of Oklahoma's 1985 national championship team.

"Our hearts are truly heavy for the entire Winchester Family. Mike was a former Sooner student athlete as was his son James/daughter Carolyn," the school's athletic director, Joe Castiglione, tweeted Tuesday afternoon. He added "daughter Becca was also a student athlete. Please keep this beautiful family in your prayers" in a later tweet.

Associated Press reporters Ken A. Miller and Tim Talley contributed to this report.