SPRINGFIELD, Mo. -- Police in Springfield are still searching for the motive of the gunman who killed four people, including a police officer, before fatally shooting himself.

The shooting happened Sunday night at a Kum & Go convenience store in the southwestern Missouri town. Police told the Springfield News-Leader they had no prior contact with the gunman, Joaquin Roman, before the crime. Investigators also were not aware of any prior arrests or history of violent crime for Roman, 31.

"We do not yet have a motive, but figuring out those details is a priority in the investigation," police spokeswoman Jasmine Bailey said.

Detectives believe Roman acted alone. The shooting was among the deadliest in recent southwestern Missouri history.

Police said the shooter fired randomly from his vehicle several times while driving through Springfield before eventually crashing into the store, where he walked inside and opened fire.

The victims included a store employee and two men who were in the store, police said. Officer Christopher Walsh, 32, was shot and killed as he tried to rescue a victim. A second officer was injured, along with another person.