The Orlando shooter's father raised a key question about the night of the massacre

The father of Orlando shooter Omar Mateen is questioning why security at the gay nightclub Pulse was unable to stop his son before he killed 49 people early Sunday morning.

"That club should have had good security," Seddique Mateen told Sky News. "The club, [with] 300 or 400 people ... coming - they should have good security. If there was good security, he wouldn't have had this opportunity."

Seddique Mateen's comments reveal a larger question that has apparently gone unanswered during the investigation into the deadliest shooting in US history: How was Omar Mateen able to enter a nightclub while carrying a handgun and an AR-15 assault rifle?

"On Saturday, as on all nights, there was a security guard posted at the front entrance. It was unclear whether Mr. Mateen shot his way into the club or smuggled in his weapons - which included an assault rifle, handgun and lots of ammunition - and then opened fire," The New York Times reported.

It is possible to modify and hide an AR-15, as Business Insider's Alex Lockie explained: "An AR-15 fitted with a short barrel and a shortened buttstock, both easily possible, results in a deadly assault rifle that is less than two-feet long, easily concealable in a heavy coat, and it's extremely easy to get."

There is no indication, though, that Mateen was using a modified rifle or wearing a coat that would have been able to hide the gun.

According to law-enforcement descriptions of the events of the night, Mateen first began shooting inside the club a little after 2 a.m. He was quickly engaged in a gun battle with an armed security guard, who was an off-duty police officer. Other officers soon join the gun fight, forcing Mateen to retreat with hostages.

Mateen had also worked as a security guard, and had a Florida firearms license allowing him to carry concealed weapons.

New reports and witness accounts have suggested Mateen was familiar with Pulse and had visited the nightclub at least a dozen times before carrying out his attack, according to the Orlando Sentinel.

The Orlando Police Department and the FBI satellite office in Orlando did not immediately respond to requests for comment.