Police and investigators work near the area of the mass shooting at the Pulse nightclub on in Orlando, Florida on June 12, 2016. A somber President Barack Obama expressed grief and outrage at the "horrific massacre" of 50 late-night revelers at an Orlando gay club, branding it an act of terror and hate. / AFP / Mandel Ngan (Photo credit should read MANDEL NGAN/AFP/Getty Images)
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Reports: Orlando gunman visited nightclub several times
02:53 - Source: CNN
Who is the Orlando gunman? 18 videos
Police and investigators work near the area of the mass shooting at the Pulse nightclub on in Orlando, Florida on June 12, 2016. A somber President Barack Obama expressed grief and outrage at the "horrific massacre" of 50 late-night revelers at an Orlando gay club, branding it an act of terror and hate. / AFP / Mandel Ngan (Photo credit should read MANDEL NGAN/AFP/Getty Images)
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Reports: Orlando gunman visited nightclub several times
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Orlando nightclub shooter Omar Mir Seddique Mateen poses with his wife, Noor Salman, in a family photo posted online.
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Source: Orlando shooter texted wife during attack
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Inside Orlando shooter's mosque
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Files: Mateen disciplined 31 times in elementary school
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The social voice of violence
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New video of Orlando shooter emerges
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Ex-wife of gunman details violent marriage
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FBI: Orlando gunman radicalized online
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Orlando nightclub shooter Omar Mir Seddique Mateen poses with his wife, Noor Salman, in a family photo posted online.
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Did Orlando shooter's wife know about plot?
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Univision reporter Juan Carlos Aguiar goes inside the apartment of Omar Mateen, who killed 49 people at Pulse nightclub in Orlando.
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Inside Omar Mateen's apartment
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Was the Orlando shooter gay?
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Residents of Orlando light candles during the first official vigil in the wake of the deadly attack at Pulse Orlando.
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Orlando shooter cased Disney and nightclub
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'Pulse' performer says shooter frequented the club
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FBI: Shooter had been investigated
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Shooter's father: Islam had nothing to do with this
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Ex-wife of Orlando shooter speaks out
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An image taken from Omar Mateen's Myspace page. Law enforcement sources identified Mateen as the shooter in the Orlando terror attack.
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Gunman called 911, claimed allegiance to ISIS
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The Orlando Police Department released this photo of Omar Mateen, the person identified as the shooter in the Orlando terror attack.
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Officials: FBI had investigated Orlando gunman
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Story highlights

Former co-worker says killer was an angry, violent bigot

Ex-wife says suspect Omar Mateen abused her, held her hostage from family

CNN  — 

As the world mourns the 49 victims of the Orlando nightclub massacre, investigators are working to piece together a picture of the man responsible for the deadliest mass shooting in U.S. history.

What could have driven Omar Mir Seddique Mateen, a New York-born security guard who was the son o