And some of those clerks, if not all of them, must have seen him--or her.

I tell you they're in a conspiracy to shield the murderer." Britz extended a hand toward the glass partition.
"Look down this room," he said.

"The murderer, presuming it was a man, must have passed down this long aisle into the office.

Then, it was necessary to repeat the journey in order to escape.

Had there been a conspiracy, then those thirty clerks must have remained quietly at their desks while the assassin walked out of the room.