Although the information would be unclassified, it's possible that Comey's letter could have been restricted as "law enforcement sensitive". Here's a quote from Wikipedia:

Sensitive But Unclassified (SBU) is a designation of information in the United States federal government that, though unclassified, often requires strict controls over its distribution. SBU is a broad category of information that includes material covered by such designations as For Official Use Only (FOUO), Law Enforcement Sensitive (LES), Sensitive Homeland Security Information, Sensitive Security Information (SSI), Critical Infrastructure Information (CII), etc.