Story highlights Workers on Hillary Clinton's private email server expressed concerns

The FBI has sought information from the company, according to sources

Washington (CNN) Employees at the company that maintained Hillary Clinton's private email server expressed concern among themselves about the way the former secretary of state's team directed them to manage data backups after the FBI started looking into the arrangements, according to emails obtained by a senator.

In a letter obtained by CNN, Senate Homeland Security and Governmental Affairs Committee Chairman Ron Johnson, R-Wisconsin, asks Datto, Inc, the makers of Clinton's server back-up system, for information on how her emails were preserved and protected. The FBI has also sought information from the company, according to sources.

Johnson indicates that a "Clinton family company," Clinton Executive Service Corp., paid for the back-up services, operated through a device called the Datto SIRIS S2000, and that the purchase was made by Platte River Networks when the server was moved from her private residence to a New Jersey-based data center in 2013.

Photos: Hillary Clinton email controversy Photos: Hillary Clinton email controversy Hillary Clinton, while U.S. secretary of state, checks her Blackberry on a military plane in October 2011. Clinton said she used a private email account for her official work at the State Department and that she did so out of convenience. But she admitted in retrospect "it would have been better" to use multiple emails. Hide Caption 1 of 7 Photos: Hillary Clinton email controversy Clinton checks her phone after addressing the U.N. Security Council in March 2012. Hide Caption 2 of 7 Photos: Hillary Clinton email controversy Clinton looks at her phone after attending a Russia-U.S. meeting in Hanoi, Vietnam, in July 2010. Hide Caption 3 of 7 Photos: Hillary Clinton email controversy Clinton types on her phone during a visit to Brasilia, Brazil, in April 2012. Hide Caption 4 of 7 Photos: Hillary Clinton email controversy Clinton hands off her phone after arriving to meet with Dutch Foreign Minister Uri Rosenthal in The Hague, Netherlands, in December 2011. Hide Caption 5 of 7 Photos: Hillary Clinton email controversy Clinton speaks on a phone in the lobby of a Honolulu hotel before briefing reporters on the Haiti earthquake in January 2010. Hide Caption 6 of 7 Photos: Hillary Clinton email controversy Clinton looks at a phone with Assistant Secretary of State for European Affairs Philip Gordon as they wait in a Munich, Germany, conference room for a bilateral meeting with the Ukrainian President in February 2012. Hide Caption 7 of 7

In the letter, Johnson quotes from emails sent by and to employees at Platte River Networks, which indicate there was discussion about how the duration of data backups could be reduced, apparently at the direction of the Clinton Executive Service Corp.

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