Providence, Rhode Island officials thought there could be upwards of 20 victims in the Brown University shooting ... according to dispatch audio.

After the initial call for the shooting came in, first responders showed up at the campus swiftly ... and, they relayed increasingly troubling estimates on the number of potential victims.

Listen to the clip for yourself ... early on, someone says they believe there are 6 victims in the shooting -- before someone later revises by saying there could be as many as 20 victims.

One person on the line then then says, "It's officially going to be a mass casualty event ... If there's possibly 20 victims, we're going to need as much mutual aid as you can get."

Individual victims are also described in the audio ... one was shot in the back according t

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