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UFC Prez Dana White is on the receiving end of some terrifying death threats from an ex-MMA fighter who says he plans to "shoot [Dana] right in the head" -- and now cops are on the case.

It's unclear what started the beef, but the wannabe fighter -- Dan Quinn -- has gone off the hinges, leaving voicemail messages threatening to blow up Dana's offices and put a bullet in the back of his head.

Quinn's fighting career spanned nine years from 2001-2010. He never fought in the UFC, but did face off against a couple of future UFC fighters, including 2004 heavyweight champ Frank Mir (Quinn got beat).

Quinn has made rambling threats against White for several years on YouTube, but they escalated recently after Quinn somehow got Dana's private work number and left chilling messages.

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In the voicemails, obtained by TMZ, the delusional Quinn accuses White of sending in henchmen to kill him, claiming, "I just want you to know, since you're bringing that to me, you should be worried about your own safety, bitch."

Quinn adds, "I'm gonna shoot you right in the head motherf**ker."

The voicemails get even weirder -- one from a guy (presumably Quinn) speaking in an Irish accent and calling himself "Paddy O'Neill." The "Irish" interloper then makes several threats.

We're told White has never met or spoken with Quinn -- sources say the Prez never ever heard of the dude until last week, when the voicemail threats got crazy.

White's security team turned the voicemails over to the Vegas PD, which is currently investigating.