COLUMBUS, Ohio (CNN) -- A gunman opened fire Wednesday night during a heavy metal concert in Columbus, killing four people -- including at least one member of the band -- and critically wounding at least two others before a police officer shot and killed him, according to Columbus police.

The attacker leapt onstage as the group, Damageplan, performed, and began "targeting members in the band," Sgt. Brent Mull said.

Two others were injured in the incident, although the nature of their injuries was not clear.

According to eyewitness accounts, the gunman initially shot the band's guitarist, Dimebag Darrell.

Darrell, 38, and his brother, Vinnie Paul, 40, formed Damageplan after Pantera -- a group they formed in the 1980s -- broke up last year. Their father is Jerry Abbott, a country and western songwriter and producer.

Police were notified of the shooting at the Alrosa Villa nightclub on the city's north end around 10:18 p.m. ET, Mull said. A uniformed police officer who was in the area slipped into the venue from a back entrance, confronted the gunman during the rampage, killing him.

"If not for the officer who showed up, there would have been more dead," Mull said.

Police have roped off a huge area of the nightclub's parking lot, as 60 detectives question hundreds of witnesses. Police have brought in buses to keep the witnesses warm as they wait to be questioned.

Calvin Bota said he saw the shooting from the mosh pit right in front of the stage, shortly after the band began playing.

"Somebody came -- I don't know where they came from, out of the audience or whatnot -- but they come onto (the) stage and ... he shot the guitarist at first, fired a couple of other shots and then he hid behind the stage a little bit," Bota told CNN affiliate WSYX.

"Everybody started scattering, you know, there's mayhem everywhere. And then a police officer came into the building, you know, came in professional with his gun raised and then he proceeded to shoot the guy."

Gerald Caudill said the gunman shot the guitarist at least four times.

"I was up close to the stage and I just saw some guy run up on the side of the stage and I heard some shots and I saw (drummer) Vinnie Paul and somebody jumped on top of Vinnie, and the guy just stayed around the stage and started shooting other people," Caudill said.

"I saw wounded people all over the place, out in the parking lot, inside," he said. "It just didn't sound like gunshots or anything, it didn't occur to me that something like that was happening."