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The owner of the convenience store where Alton Sterling was killed said Wednesday he believes the man was "murdered" by the two white Louisiana cops who shot him.

The U.S. Justice Department is now leading a civil rights investigation into the death of Sterling, 37, who was shot multiple times early Tuesday outside Abdullah Muflahi's Triple S Food Mart in Baton Rouge, the state capital.

Related: 'Full of Joy': Family Mourns Alton Sterling

Graphic cellphone videos — including one filmed by Muflahi himself (above) — appear to show Sterling being tackled, shot multiple times and pinned to the ground.

Authorities claim Sterling was armed, but Muflahi told NBC News on Wednesday that as far as he knew, Sterling never brandished a gun or threatened the officers in any way.

Related: Alton Sterling Shooting by Baton Rouge Police Sparks Outrage, DOJ to Investigate

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"They shot him three times already, then they shot him another three times," Muflahi said.

"I believe he was murdered," Muflahi said.

Muflahi said he had no idea why police arrived at his store to arrest Sterling, who sold CDs outside — and neither did Sterling, he said.

"He just wanted to know what was going on. Why are they coming to arrest him?" Muflahi said. "He was asking them: 'What'd I do wrong? What's going on? What'd I do wrong? Why you messing with me?'"

Photos: Protesters Speak Out After Baton Rouge Police Shooting

Baton Rouge police said a caller to 911 claimed that Sterling was acting threatening with a gun. But Muflahi said he'd known Sterling for five or six years and had never seen him get into a fight or even an argument.

"He was a nice guy, always smiling, always happy, always joking around with people," Muflahi said.

"He sits right outside the door. If there was anything that had happened, I think I would've heard it. I didn't hear nothing," Muflahi said.

Muflahi said he did see the officers remove a weapon from Sterling's pocket after they shot him. As a convicted felon, Sterling wouldn't have been permitted to have a gun, but Muflahi and others who knew him said he kept one to protect himself from robbers.

Muflahi stressed that even the cops indicated that the gun was still in Sterling's pocket after he'd been shot, indicating that he'd never had a chance to pull it during the confrontation.

"I don't think there was any way that he would've reached for it," Muflahi said. "And if he would've reached for it, his hand would've still been in his pocket, or the gun would've been in his hand after they had killed him."

Muflahi said he was shocked — "it was like a nightmare."

"Seeing your own friend getting shot in front of you, it's horrifying. It's scary," he said.

"We just want the truth to come out," he said. "We want justice. That's it."