It was the officer that aproached him. for walking in the middle of the street.
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The officer who was in his police vehicle and driving on the road (I think it goes something like - road=cars; sidewalks = people; except in cases when there is no sidewalk then you walk on the side of the road to avoid accidents) did not approach Brown in any report that was aggressive. The report was that he told the men to get off the middle of the road and continued driving past them.
Walking down the middle of a street poses a danger to drivers.
The report indicated that the officer drove past them - end of that interaction. The officer clearly had no interest in the two men who continued to walk down the middle of the street at that time - and the two suspects had no interest and felt no threat from the officer who had just warned them to get off the middle of the street. The suspects appeared to have no fear of the officer as they did not heed his warning and continued their walk.
A call came in detailing the clothing of the robbery suspect and it was then the officer realized that the description of the suspects appeared to match the dispatch.
The officer backs up his car to identify the suspects for sure and promptly calls for back up. He tries to open his door and it is slammed shut. The altercation/struggle in the car begins.
The first verbal altercation has nothing to do with the second altercation, except as to understand why the officer recognized the two men so quickly after the repot and drove back to them to confirm their identity as the robbery suspects.