Shooters in the plural. If you monitor the police announcements, you will see they captured a second gunman alive after a shootout with SWAT —”shooter in custody.” This was a couple of hours before they blew up X. All references throughout the evening were to terrorists in the plural, stating maybe as many as 4 to 5.
Definitely, there was a second shooter and he has now been disappeared.
Some claim that it was a person open carrying. At least that was Obamas explanation.