How the Orlando Killer Omar Mateen Got His Guns

The call at 911 in Orlando came in at 2:28 am, but the caller hung up without saying anything.

The 911 operator followed the standard procedure of calling back and Omar Mateen answered while in the midst of committing the worst mass shooting in American history, a senior law enforcement source tells The Daily Beast.

The source further reports that Mateen had purchased a Sig Sauer .223 caliber assault rifle at a firearms shop near his Florida home, St. Lucie Gun Sales, on June 4 and then a Glock 17 at the same store on June 5. Mateen had returned to the store a third time on June 9 to buy magazines for his weapons. The store is a federally licensed firearms dealer. Under law, the seller would have had to notify the Federal Bureau of Investigation of Mateen's purchase so that his name could be checked against the National Instant Criminal Background Check System, or NICS.

Mateen was actually listed on two federal watch lists, a U.S. official tells The Daily Beast: The Terrorist Identities Datamart Environment, which contains classified information, and the Terrorist Screening Database, which is the FBI's central watchlist. The gun background check would have run Mateen's name against that second database, but he had been removed from it in 2014. The sales were approved and early Sunday morning he used the weapons to fire round after round after round at defenseless people at the Pulse nightclub. Mateen left a third weapon, a revolver capable of firing only a mere six shots, in his van.

An FBI spokesman didn't respond to a request for comment about whether the gun seller made the required check. However, it's unlikely it would have raised any red flags.

Even if it had, Mateen still could have purchased the weapons legally. The U.S. official says that, at most, the FBI would be alerted that he was trying to buy the weapons and perhaps agents would have watched Mateen more closely.

Instead, it was Mateen who reached out to law enforcement—on the night of his rampage. As paraphrased by the senior law enforcement source, Mateen told the 911 operator that, “I’m doing this to protest the U.S. bombing in Syria and Iraq and the killing of women and children… I’m doing this in solidarity with the Tsarnaev brothers and Moner Abu-Salha.”

Tamerlin and Dzhokar Tsarnaev committed the Boston Marathon bombing in 2013. Mateen’s claim to know the brothers had been reported to the FBI by his co-workers at G4S Security. The FBI had investigated and found no actual connection.

Moner Abu-Salha was the first American suicide bomber in Syria. He also spent time in Fort Pierce, the Florida town where Mateen lived with his wife and son. Mateen had also claimed to have a link to Abu-Salha and that had led to a second FBI investigation. The FBI found no significant connection in this instance, either.

The source says that at some point Mateen did enroll in the online Fundamental Islam Knowledge Seminary, founded by Marcus Robertson of the Bronx.

Robertson adopted the name Abu Taubah and moved to Florida in 2010. Abu Taubah was convicted of weapons and tax charges but has since been released. He is said to have vehemently condemned homosexuality. But Mateen does not seem to have mentioned Abu Taubah during the 911 callback or during several other conversations with 911 that followed.

Read more coverage of the massacre at the Pulse nightclub

CORRECTION, JUNE 13, 10:48 PM: An earlier version of this story stated that Mateen purchased a Sig Sauer .221 caliber assault rifle. The correct caliber is .223.