"If the FBI is denied the ability to gather evidence through compulsory means, Secretary Clinton and her aides have enormous leverage to negotiate extraordinary concessions in exchange for voluntary cooperation," he wrote.
Grassley said that his concerns were "magnified" by the lack of detail in Comey's letter.
"Needless to say, it is disturbing if political appointees at the Department of Justice are inappropriately withholding compulsory tools from the FBI, forcing agents to depend on a subject or target’s consent to gather evidence, which undercuts the FBI’s ability to do its job and ultimately obtain justice," Grassley wrote.
He called on Comey to provide additional context for his announcement, calling the announcement "not fair to Congress, the American people, or Secretary Clinton."
Grassley included a list of questions for Comey about the content and relevance of the emails, as well as whether the FBI requested the ability to use compulsory methods in its investigation of Clinton's server or its separate investigation into the Clinton Foundation.
Read the full letter below: