State Department officials can't find the emails of Bryan Pagliano, the aide who set up Hillary Clinton's private server.

The agency is unable to search Pagliano's work computer for the emails, which were requested by the Senate Judiciary Committee, because the FBI has taken the computer into custody, according to a letter made public by Sen. Chuck Grassley's office Friday.

"The Department has searched for Mr. Pagliano's email .pst file, and we have not yet located one that covers the time period of Secretary Clinton's tenure," Alec Gerlach, a State Department spokesman, told the Washington Examiner Monday. "We are continuing to search for Mr. Pagliano's emails which the Department may have otherwise retained."

A .pst file stores emails and other items in Microsoft programs like Outlook.

Pagliano's work-related emails became a top priority for the committee after the IT specialist invoked his Fifth Amendment rights and refused to answer questions in separate congressional inquiries, including during an appearance before the House Select Committee on Benghazi.

"Mr. Pagliano was reportedly paid by former Secretary Clinton for his work on her non-government server while a State Department employee, though he apparently did not declare any such income on his financial disclosure forms after entering the department," Grassley noted in his letter to Secretary of State John Kerry on Friday.

Grassley, who chairs the Judiciary Committee, said the search for Pagliano's records was the panel's "highest priority request" after praising State Department staff for their efforts to provide other documents and testimony.

"However, the department has informed the committee that it is unable to locate any copy or backup of the '.pst file' containing Mr. Pagliano's official State Department emails," Grassley noted.

"As I mentioned to FBI Director Comey during Wednesday's FBI oversight hearing, the department also informed the committee that the government computer system Mr. Pagliano is believed to have used is now in the possession of the FBI," the Iowa Republican continued. "As a result, the department is searching for, and has located a limited number of emails in the files of other State Department employees who communicated with Mr. Pagliano."

Pagliano's refusal to answer questions earlier this year about his work on Clinton's private server raised eyebrows given the fact that other officials with knowledge of the arrangement complied with congressional requests.

An FBI probe is presently looking at whether the email network violated laws meant to protect sensitive information.

Hundreds of emails made public by the State Department this year have been marked classified, raising questions about who in the agency authorized the use of a server that potentially compromised classified material.