"Unmarked vehicles, disappearing people off the streets?"

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Why they are unmarked vehicles 101
Portland Police respond as people break in and ignite fire in Portland Police Association office in North Portland (Photo) - 07/19/20
On Saturday, July 18, 2020, crowds of people gathered throughout Portland for various demonstrations. One group began at Peninsula Park and walked to the Portland Police Bureau's North Precinct blocking streets as they went. At around 8:35 p.m., hundreds gathered on the south side of the precinct in a parking lot. Some people tampered with gates, broke patrol vehicle windows, and vandalized patrol vehicles. People taunted officers as they arrived to work.
Why the "protesters" are detained nowhere near a federal building 101.
At about 10:45 p.m., people broke into the PPA office and within a few moments ignited a fire inside. As Portland Police arrived, many began to flee to the east.