(CNN) -

What really happened to Michael Brown?

We know a police officer in Ferguson, Missouri, fatally shot the unarmed 18-year-old, but police and witnesses have given different accounts of what happened in the moments before the shooting. The confusion has fueled protests in Ferguson and, earlier in the week, strong police response in the streets of the St. Louis suburb.

Here are five key questions about the incident:

How did it start?

Brown and Dorian Johnson, 22, were walking in the middle of the street, en route to either Brown's grandmother's house (according to Brown's mother and grandmother) or to Johnson's house (according to Johnson), when a Ferguson police officer confronted them.

Officer Darren Wilson, who is white, told the young men either "Get the f*** on the sidewalk" or "Get the f*** out of the street," according to Johnson's accounts to CNN and other news outlets.

The young men replied that they were "not but a minute away from our destination, and we would shortly be out of the street," Johnson told CNN.

The officer drove away but stopped and backed up, almost hitting the pair, Johnson said. He said he wasn't sure what prompted the officer to return. Johnson told MSNBC the officer said something to the effect of "What'd you say?"

"We were so close, almost inches away, that when he tried to open his door aggressively, the door ricocheted both off me and Big Mike's body and closed back on the officer," Johnson said.

A caller to the St. Louis radio program The Dana Show, on Radio America, gave what she said was the officer's version of events. Her account accurately matches what Wilson has told investigators, a source with detailed knowledge of the investigation told CNN.

The caller, who identified herself only as "Josie," said that Wilson told her that the two young men were walking in the street and he rolled down his window and told them to get out of the street.

Was there a struggle?

Yes. Everyone agrees on this point, and it's one of the few aspects of the shooting that police have attempted to detail, though the official explanation has spurred many questions they have yet to answer.

The preliminary investigation showed that the officer tried to exit his vehicle, but Brown pushed him back into the car, "where he physically assaulted the police officer" and struggled over the officer's weapon, St. Louis County Police Chief Jon Belmar said.

A shot was fired inside the police car, Belmar said. After the incident, the officer was taken to an area hospital, where he was treated for a "swollen face," according to Ferguson Police Chief Thomas Jackson.

In Josie's version, Wilson may have heard a call about a strong-arm robbery and saw the young men carrying something that might have been stolen cigars. According to Josie, when Wilson tried to get out of his car, Brown pushed him back in, and then punched the officer in the face. It was then that Wilson reached for his gun, Josie told the radio show.

Brown grabbed Wilson's gun, and the officer pushed it away and the gun went off, Josie said.

The stories don't jibe with at least three witness accounts.

Johnson claims the officer grabbed Brown by his neck, and Brown tried to pull away, but the officer kept pulling Brown toward him, he said.

The officer drew his weapon, and "he said, 'I'll shoot you' or 'I'm going to shoot' " and almost instantaneously fired his weapon, hitting Brown, Johnson said.

Witness Tiffany Mitchell was picking up coworker Piaget Crenshaw for their jobs when she saw Brown and the officer "tussling through the window." Mitchell and Crenshaw said Brown appeared to be trying to pry himself from the officer's grasp. Brown had his hand on the police cruiser, trying to push himself away, Mitchell said.

Was Brown armed?

No. Again, this is undisputed.

Every casing found at the scene came from the officer's gun, Belmar said, and witnesses say that after the officer initially fired, the two young men took off running.

"I saw the officer proceeding after my friend, Big Mike, with his gun drawn, and he fired a second shot, and that struck my friend, Big Mike," Johnson said. "And at that time, he turned around with his hands up, beginning to tell the officer that he was unarmed and to tell him to stop shooting. But at that time, the officer was firing several more shots into my friend, and he hit the ground and died."