BOULDER, Colo. (KDVR) — Four days after that mass shooting in boulder investigators say they still do not have a motive, a reason for that location- the grocery store or why Monday was chosen as the day for the attack.

Authorities did reveal that the suspect Ahmad Al Aliwi Alissa passed a background check to purchase the gun he used to allegedly kill ten people. That purchase took place 6 days before the rampage.  

While a motive has not been determined yet, investigators did say they are taking a deep dive into the suspect’s background. 

Outside the King Soopers, the FBI’s Mobile Command unit remained in place. Investigators said there is an enormously complex crime scene to process.  

“If you picture a supermarket, picture all the shelves, the products. They’re going through every single shelf. Pulling everything off the shelves. Looking in the walls,” said Boulder County District Attorney Michael Dougherty. 

Investigators are trying to find among other things bullet fragments in this huge grocery store the size of a football field.  

The DA said the investigative and court processes could take a year or more to complete.  

The scene itself is posing unique challenges. 

“Just the vastness of the scene. You’re talking about a huge supermarket obviously. A huge parking lot. Numerous vehicles. It’s one of the most complex scenes I’ve ever worked,” said Boulder Police Chief Maris Herold.  

Herold and a team of investigators have walked the entire crime scene.  

Investigators are still reviewing surveillance video and talking to many witnesses. That’s another reason this will be a slow, methodical investigation.  

“Collecting that evidence from a scene as chaotic as that was with that many moving parts and that many people moving around can be an enormous task in and of itself,” said CU Denver Associate Criminal Justice Professor Paul Taylor.  

Determining a motive is the primary focus now. Investigators are now taking a deep dive into the suspect’s history and possible ties to terror groups.  

“It will be something haunting for all of us until we figure that out, and like someone said, sometimes you just don’t figure these things out. I am hoping that we will,” Herold said.  

Why this shooting happened is the question on so many people’s minds.