Benke said the investigation started around 1:30 p.m. Tuesday when a resident reported finding the substance in his mailbox. By late afternoon, investigators said tests concluded the substance was not hazardous.
But as of Wednesday, he said investigators have not determined exactly what the substance was.
No one has reported additional discoveries of the substance and residents no longer need to be concerned about their mailboxes, Benke said.
Anyone with information on the substance should contact Charleston County Consolidated Dispatch at 843-743-7200.