In a letter to the deputy attorney general, Sen. Chuck Grassley wrote that his staff had reviewed the memos and asked for more information on the timing of their classification.
Comey testified in June that he gave some of his memos to a Columbia University professor and that he had written the memos specifically to avoid including classified information.
"My thinking was, if I write it in such a way that I don't include anything that would trigger a classification, that'll make it easier for us to discuss, within the FBI and the government, and to -- to hold on to it in a way that makes it accessible to us," Comey told senators.
The professor, identified as Daniel Richman, told CNN in July that none of the memos
given to him was marked as classified.
CNN has reached out to the Justice Department for comment but has not yet received a response.
In the letter, Grassley cites an interview Richman gave to Fox News saying that he received four memos from Comey.
"If it's true that Professor Richman had four of the seven memos, then in light of the fact that four of the seven memos the Committee reviewed are classified, it would appear that at least one memo the former FBI director gave Professor Richman contained classified information," Grassley writes.
Judiciary Committee investigators have not determined which of the seven memos Comey gave Richman and Richman has declined to provide the memos to committee staff, Grassley said. Richman also refused a request in October to participate in a voluntary transcribed interview with committee staff, Grassley said.
Grassley in the letter asks the DOJ which letters Comey sent to Richman and when he sent them, and whether any of them contained classified information.
Comey testified in June that he had a habit of documenting his conversations with the President, and told senators that some memoranda and emails were classified and written on a classified computer, while others were unclassified.