During an interview with ABC News, Hillary Clinton apologized for using a private e-mail server during her time as secretary or state. Here are past statements where the presidential hopeful neglected to take personal responsibility for the controversy. (The Washington Post)

A technology subcontractor that has worked on Hillary Rodham Clinton’s e-mail setup expressed concerns over the summer that the system was inadequately protected and vulnerable to hackers, a company official said Wednesday.

But the concerns were rebuffed by the company managing the Clinton account, Platte River Networks, which said it had been instructed by the FBI not to make changes. The FBI has been reviewing the security of the e-mail system.

The subcontractor, Datto, which specializes in backing up data, had not been aware that it was handling Clinton e-mails until media reports in August noted Platte River Networks’ involvement with the controversy surrounding the former secretary of state’s e-mails.

Datto officials, worried about the “sensitive high profile nature of the data,” then recommended upgrading security by adding sophisticated encryption technology to its backup systems, said the Datto official, who requested anonymity to discuss an issue involving a client.

The Datto official said there is no evidence the company’s systems had been attacked.

A Platte River Networks spokesman acknowledged receiving upgrade requests from Datto.

“It’s not that we ignored them, but the FBI had told us not to change or adjust anything,” the spokesman, Andy Boian, said.

Boian said, however, the company did not take Datto’s concerns to the FBI.

An FBI spokeswoman declined to comment.

The concerns expressed by Datto reflected worry that the system, which was still in use for the Clintons’ personal office in August, could have been vulnerable to hackers who targeted it for its new notoriety amid the swirling controversy.

They also suggested potential security gaps dating back to when the company was first hired, in May 2013, possibly putting at risk material from Clinton’s time as secretary of state that the company believes it might have stored since that time.

And it points to a larger question, being posed by Republican lawmakers, of whether Clinton, the Democratic front-runner, jeopardized national security by using a private system that lacked top-level security protocols that would be expected of a high-level government official.

Catch up on the controversy and read the emails

Both Platte River, based in Denver, and Datto, in Connecticut, typically service small-business clients and have limited experience with sensitive government information.

Clinton and her aides have said that government secrets were never put at risk. Asked about Datto’s concerns, Brian Fallon, a campaign spokesman, pointed to the company’s assurance that Clinton’s data had not been hacked.

A Clinton aide, requesting anonymity to discuss a sensitive matter, said the Clintons instructed Datto and Platte River to fully cooperate with the FBI. The Clinton aide also said the arrangement between Platte River Networks and Datto was “negotiated by the two companies, not the Clintons.”

Officials have said that Clinton is not a target of the FBI inquiry. Investigators took possession of a server from the Denver-based Platte River server in August, and Datto told The Washington Post this week that it had arranged to hand over hardware used by the company to back up e-mail data in its cloud.

The probe has exposed a disagreement between Platte River and Datto about whether the subcontractor’s backups are likely to have stored material from the time Clinton served as secretary of state. Datto believes it is possible; Platte River has said it is unlikely.

A security breach would have been less serious if Datto held none of Clinton’s government records.

The dispute also relates to the likelihood that investigators might be able to retrieve from Datto’s equipment at least some of the 31,000 e-mails from Clinton’s time as secretary that she deemed personal and has since deleted.

Clinton in December handed over about 30,000 e-mails to the State Department that she said were related to her official duties. But some GOP lawmakers have questioned whether Clinton had deleted additional work e-mails along with her personal correspondence and have called for the deleted e-mails to be recovered.

Carol D. Leonnig contributed to this report.

This story has been updated to include a quote from a Clinton aide addressing the Clintons’ role in the negotiations between Datto and Platte River Networks.