More details released about critical 12 minutes of Las Vegas mass shooting

With the Mandalay Bay Resort and Casino being so vast and hotel security assigned to another event, police officers across the street were able to respond faster to the reports of shooting.
1:40 | 10/19/17

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Transcript for More details released about critical 12 minutes of Las Vegas mass shooting
Next to the massacre in las Vegas. Tonight, we have new reporting here about those fateful 12 minutes, and when police finally arrived. And which teams were actually able to get there first? ABC's Matt Gutman again tonight. Reporter: Tonight, new details about those 12 critical minutes as police responded to the Las Vegas rampage. ABC news has learned that there were police officers in the mandalay bay hotel securing another event when Stephen paddock opened fire. But the hotel complex is so vast that, police say, officers who were across the street at the country music concert got there faster. @Unning through the streets in that rain of bullets. One of the things that's most striking as you see this from the air, is that vast distance from that suite up on the 32nd floor down to that concert venue down below. 12 minutes after security officer Jesus campos was wounded and radio'ed for help. In his first interview, campos telling Ellen DeGeneres how he'd been sent to investigate an open door in the stairwell. As that door is closing, and it's so heavy, it will slam. I'm walking down this way, and I believe that's what caught the shooter's attention. As I was walking down, I heard rapid fire. Reporter: The gunman shooting through his hotel room door, hitting campos in the leg. With this new timeline, the remaining mystery is, why did Stephen paddock do this? Now, police tell me that he was so secretive that we may never know. David? Matt Gutman again tonight.

This transcript has been automatically generated and may not be 100% accurate.

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