Only if he went through the parking garage. The lobby was evidently staffed 24 hours a day. Though it is feasible that a current employee would allow a former employee to enter, an employee belonging to a more professional company would not permit this.
I also suspect that the most likely route of access was through the garage. The killer had to have bypassed the front desk. Just last summer police interviewed employees there in connection with Teixeira's bank robbery investigation. Those employees identified him in surveillance photos. That's another reason why he couldn't have entered through the front door.
But still, you'd expect that security cameras were covering the garage 24/7.