So they’d need the actual codes or keys to even get in.
If all three of the main actors worked in the building, they’d all have access. One of the three was *involved* with both of the other two, and a dispute arose which left two dead and one “suspect.”
Other scenarios are available, of course, in the absence of other information, but this is a really easy, obvious one.
Don’t know if they were inside. All the article says is at. They could have been walking to/from their car.