A powerful senator is demanding FBI Director James Comey explain his decision Sunday to recommend for a second time against prosecuting Hillary Clinton for mishandling classified information through the use of her private email system for official business.

In a letter to Comey, Senate Homeland Security and Governmental Affairs Chairman Ron Johnson, R-Wis., asked for details on Comey's scuttling of the latest probe, which had been triggered when a fresh batch of 650,000 emails potentially involving Clinton turned up in an unrelated investigation.

“Your two most recent letters to the Committee leave several unanswered questions about this new material and the FBI's review of it,” Johnson wrote in the letter sent Monday.

Comey called Clinton “extremely careless” in her handling of sensitive emails at a July 5 news conference in which he said he would not recommend that the Department of Justice seek an indictment for violation of the Espionage Act. The discovery of a massive new trove of emails, turned up in an unrelated probe of former Congressman Anthony Weiner, D-N.Y., prompted Comey to announce last month the probe had been reopened.

Weiner, who is suspected of sending lewd messages to a 15-year-old girl, is the estranged husband of Clinton’s top aide, Huma Abedin.

Johnson wants Comey to explain his conclusions, given that the FBI found in its initial investigation that 81 email chains with 193 individual emails on Clinton's email system contained information classified at the time the emails were sent or received. The investigation also showed Clinton emailed directly with President Obama while outside the country, he wrote.

A Sunday report in the New York Post revealed Clinton ordered her maid, who did not have any level of security clearance, to access and print classified information, and, in the process, entered the sensitive compartmented information facility in her home.

“Did the FBI examine these facts? If so, did the FBI determine whether this individual in fact had access to classified information? Johnson wrote. “Did the FBI take into account these facts when evaluating whether former Secretary Clinton was grossly negligent in the handling of classified information?”

The senator also asked how many new emails the FBI reviewed, whether any contained classified or top secret information at the time the emails were sent or received, and, if so, who had access to the information. He also demanded to know whether the FBI continues to examine matters relating to other current or former State Department employees relating to the mishandling of classified information or the destruction of federal records.