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.