Washington, August 5 (RHC)-- The Federal Bureau of Investigation (FBI) is investigating the security of the private e-mail server that Hillary Clinton used when she was secretary of state.

The FBI has contacted in the past week a technology firm based in Denver, Colorado, that helped manage the unusual system, The Washington Post reported on Tuesday, citing two government officials.

The domestic intelligence and security agency also contacted Clinton's lawyer, David Kendall, with questions about the security of a thumb drive in his possession that contains copies of work emails Clinton sent during her time as the top US diplomat.

"The government is seeking assurance about the storage of those materials. We are actively cooperating," Kendall confirmed. The FBI began inquiring about Clinton's email system after the intelligence community's inspector general referred the issue to the U.S. Justice Department in July.

Senior intelligence officials have expressed concern that some sensitive data that she sent or received was not in the government's possession and could be "compromised." A lawyer for the Denver company, Platte River Networks, declined to comment, as did multiple Justice Department officials.

In 2013, Hillary Clinton hired Platte River to maintain the server's data after the system crashed days after New York was hit by Hurricane Sandy in October 2012, while Clinton was secretary of state.

The inquiries are revealing new information about Clinton's use of the system and the lengths to which she went to install a private network of communication outside government control.

Critics say Clinton's private server arrangement made her communications inaccessible to government investigators, congressional committees and courts seeking public records from the U.S. State Department.

The private server, as well as other recent scandals, has emerged as a major obstacle in Clinton's presidential campaign for the Democratic nomination.

More Americans now view the Democratic front-runner negatively than they did a month ago, a new survey reveals. Just 37 percent of all Americans have a positive view of Clinton, versus 48 percent who have a negative view, according to an NBC News/Wall Street Journal poll released Monday.

The figures show a sharp decline in her popularity since June, when she had a 44 percent positive and 40 negative rating.