Witnesses encountered Fake Secret Service Agents in at least two locations in Dealy Plaza Immediatley after the shooting.
DPD Officer J M Smith encountered one behind the picket fence atop the grassy knoll.

Below Volume VII-535.

Head of the Dallas Secret Service Forrest Sorrels encountered another fake Secret Service Agent at the back door of the TSBD after returning to the scene from Parkland Hospital 20/25 minutes later.
DPD Officer D V Harkness also met Fake Secret Service Agents.

Below-Volume VI-312.

The Warren Report tells us that were NO Secret Service Agents on the ground in Dealy Plaza that day.
They were all in the motorcade & Stayed with the motorcade all the way to Parkland Hospital.
ALL Secret Service Agents working the President's visit were Identified by "Lapel Pins".

Those Lapel Pins CHANGED Shape/Color every day.
On 11/22/63 the Lapel Pins were Red/White Rectangular.

Those Lapel Pins were shown to every Dallas Officer who worked the Presidential visit to Dallas.

Each Agency had their Own I D systems. (see above)
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