Dallas Assassinations: Police Incredibly Calm Under Pressure

TMZ obtained this video of at least 9 Dallas cops taking what appears to be a possible person of interest into custody immediately after Thursday's mass shooting ... and despite crushing pressure and emotion, they keep their cool.

The man in question clearly captured the attention of police because he's wearing a bulletproof vest. We don't know if police kept the man in custody or if he had any connection to Thursday's shootings.

The scene was almost shockingly calm considering what had unfolded only moments before, but there's still tension ... you hear bystanders crying out "Do what they say!!" as the man cautiously approaches.