Police are investigating the armed robbery of an employee outside a Sandy Springs public library.

The robbery happened as a courier was making his rounds at the branch on Mount Vernon Highway the morning of July 23. The courier was picking up some cash when an armed man robbed him, Sandy Springs police said.

"He parked the vehicle, left the vehicle unlocked and walked in, only intending to be in the building a short time," Sandy Springs police Capt. Steve Rose said.

Rose said the courier saw a man rummaging through his van and confronted him.

“He approached him, yelled at him.  The man turned out and actually pointed a weapon at the employee," Rose told Channel 2’s Mike Petchenik.

Then, the courier ran back into the building, which was placed on lockdown as police searched for the gunman.

"Right now, we're working on a vague lookout but also any possibilities he may have had knowledge this is what the courier truck does," Rose said.

It is unclear what the thief took from the van.