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Get up to speed on the Ferguson shooting investigation

After months of anticipation, a grand jury's decision on whether to indict a Missouri officer for fatally shooting an unarmed black teenager could come at any moment.

And when it does, anything could happen in the tinderbox that is Ferguson, Missouri.

But how did we get to this point? Here are six things to know to get up to speed on the case:

How did this start?

On August 9, 18-year-old Michael Brown and his friend Dorian Johnson were walking in the middle of a residential street when a white officer, Darren Wilson, pulled up and told them to get out of the road.

That's where the stories quickly diverge.

According to Johnson, he and Brown told the officer they were almost at their destination and would be out of the street shortly. But Johnson said the officer grabbed Brown by the neck and drew his gun, eventually shooting Brown.

\"At no point in time did they struggle over the weapon because the weapon was already drawn on us,\" Johnson said.

By contrast, a Wilson family friend identified as \"Josie\" told local radio station KTFK that Brown started a physical altercation with the officer.

\"Michael just bum rushes him, and just shoves him back into his car, punches him in the face and then of course Darren grabs for his gun and Michael grabs the gun,\" Josie said. \"At one point he's got the gun totally turned against his hip, and then he shoves it away and the gun goes off.\"

A source told CNN that Josie's account matches what the officer told investigators.

What was Brown doing when he was shot?

Both sides agree that Brown ran and then turned back -- the question is whether he turned to attack or surrender.

\"He was running for his life and just got shot and turned around and didn't try to reach for anything,\" witness Piaget Crenshaw said. \"He put his hands in the air being compliant, and he still got shot down like a dog.\"

Josie gives a different explanation.

\"All of a sudden (Brown) just started to bum rush him. He just started coming at him full speed so (Wilson) just started shooting and he just kept coming,\" Josie said. \"The final shot was in the forehead and then he fell about 2, 3 feet in front of the officer.\"

How many times was Brown shot?

The official autopsy, as published by the St. Louis Post-Dispatch, said Brown suffered six gunshot entrance wounds.

The wounds included a shot to the hand at close range, according to an autopsy analysis reported by the Post-Dispatch. That detail could support Wilson's claim of a struggle inside his patrol car.

But that doesn't explain why Wilson continued shooting, including at least one shot to Brown's head.

A private autopsy conducted for the Brown family showed the teen had been shot at least six times, including twice in the head.

Why is a grand jury involved?

The grand jury is deciding whether Wilson should be charged with any one of several possible crimes.

Those possible charges include first-degree murder, second-degree murder, voluntary manslaughter or involuntary manslaughter, said Ed Magee, spokesman for the prosecuting attorney's office.

The grand jury can issue an indictment on any of those four charges, and it also has the option of adding a charge of armed criminal action, authorities said.

At the same time, the grand jury will consider Missouri's statutes for self-defense and police use of deadly force.

How many votes are needed for an indictment?

Different states have different numbers, but there are 12 members on the Missouri grand jury. Nine of the 12 must vote yes to indict someone on a charge.

The grand jury proceedings are conducted in secret. The grand jurors' races and ethnicities are also kept under wraps.

Why do we think the grand jury's decision could come soon?

Though the grand jury has until January to issue its ruling, the prosecutor's office has said a decision could come in mid-November.

CNN's Eliott C. McLaughlin, Sara Sidner, Sara Weisfeldt and Michael Martinez contributed to this report.

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