The incident stemmed during a gang fght that broke out after a car crash at Juniper Drive and Winterberry Court.
An Arlington police source said the off-duty Houston officer ordered Reynolds to the ground, when he pulled out a gun. Reynolds was shot 6 times.
Police called for assistance after an angry crowd gathered around the scene of the shooting.
Neighbors said the area has seen a lot of violence in the last two weeks.
Worried violence would spill over into the school day, Mansfield ISD increased security at Summitt high on Tuesday.
NBC DFW's Grant Stinchfield contributed to this report.