Which staircase doors must be locked?

The office building where I work has two staircases. One extends from the basement/garage to the top floor and exits on the first floor into a vestibule. The other extends from the first floor to the top floor and exits onto the outside porch at the first floor.

The first staircase is locked on every floor but the first floor and garage level. The second staircase is open on all upper floors with a magnetic lock on the first floor exit. We have been told that doors must be locked in the first staircase because of fire code, but that it is ok for the second staircase to be unlocked.

As our office is split over 2 floors, there is a fair amount of up-and-down traffic. It would be much more convenient for us to use the first staircase, but we can't because the doors are locked.

One of my co-workers read through the "egress" section of the fire code and cannot find anything that seems to match our situation that would require the doors to be locked.

Can anyone explain this situation, and point to the relevant bit of fire code or city code that requires the doors to be locked?

Thank you.