Early reports on social media of shots being fired at the busy casino and hotel were false and there were no injuries, the Las Vegas Police Department said on Twitter.
The Bellagio is home to several high-end jewelry and watch stores, including Tiffany and Harry Winston.
It began to transpire that men dressed with pig masks entered the hotel to rob a Rolex store. Parts of the casino property were closed as police investigated.
The suspects wore character masks, but Lt. Hank could not confirm the specifics of the mask. The Bellagio remained on lockdown for about 30 minutes after the incident. There were also other reports of tourists hearing gunshots and that there was an "active shooter" in the area.
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Bellagio is a casino, resort, luxury hotel and resort on the Las Vegas Strip in Paradise, Nevada.
Police said the three suspects fled the robbery into a nearby parking garage. They reportedly smashed the window with a sledgehammer.
At least one person was taken into custody as of 2:15 a.m. local time, as police continued to search for suspects.
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