Hillary Clinton's private attorney, who was permitted to retain copies of Clinton's emails until earlier this month, may not have had the proper security clearance or adequate tools to protect those documents.

"[I]t appears the FBI has determined that your clearance is not sufficient to allow you to maintain custody of the emails," wrote Sen. Charles Grassley in a letter Friday to David Kendall, Clinton's attorney.

Three days earlier, the FBI had taken from Kendall a thumb drive that he used to store copies of the work-related emails Clinton turned over to the State Department in December of last year.

Investigators grew increasingly concerned about the documents Clinton and her attorney still possessed after the intelligence community inspector general determined some of the emails were classified up to "top secret," the highest level of classification in government.

Grassley noted the State Department had identified classified information among Clinton's emails as early as May.

But the agency did not give Kendall a safe in which to store the thumb drive until July, the Senate Judiciary Committee chairman said.

"Thus, since at least May 2015 and possibly December 2014, it appears that in addition to not having an adequate security clearance, you did not have the appropriate tools in place to secure the thumb drives," Grassley wrote. "Even with the safe, there are questions as to whether it was an adequate mechanism to secure [top secret] material."

The Iowa Republican pressed Kendall on whether his security clearance was active when the State Department allowed him to keep the thumb drive despite knowing it contained classified information.

Grassley also demanded to know who besides Kendall was granted access to the emails at Williams & Connolly, the law firm where he works.

Officials involved in the screening process for the roughly 30,000 emails Clinton provided to State have raised questions about potential conflicts of interest involving attorneys with ties to Kendall's firm, according to media reports.

For example, Catherine Duval, the State Department attorney in charge of the agency's efforts to produce the Clinton emails to the public and to Congress, worked at Williams & Connolly before entering government service.

Duval oversaw the Internal Revenue Service's document production in the controversy surrounding missing emails sent and received by Lois Lerner, the former head of the agency's tax-exempt unit.

Intelligence experts raised concerns about interference in classification decisions from State Department officials, some of whom may have used FOIA exemptions to hide the extent of classified information among Clinton's emails.