WASHINGTON — Late last month, Hillary Rodham Clinton stood before a line of television cameras at a rural Iowa campaign stop to deny reports that she had sent sensitive information over her private email system.

“I’m confident that I never sent or received any information that was classified at the time it was sent and received,” Clinton said, dismissing claims to the contrary by federal intelligence officials as a bureaucratic dispute over what qualifies as classified.

The view from inside Clinton’s presidential campaign team was much the same: Clinton had done what she needed to do, there was nothing of real concern regarding the emails and, mostly, the whole matter was an annoyance in her efforts to win the White House.

The next week, however, law enforcement officials became interested, and the campaign’s apparent lack of concern began to turn into a sense of anxiety.

“They’re worried about it,” said a longtime Clinton adviser and confidant who agreed to discuss the mood of the campaign team only on the condition of anonymity. “They don’t know where it goes. That’s the problem.”

The controversy over her private email setup has moved into a new and, potentially, more serious phase. What had begun five months ago as a relatively narrow question about proper archiving of public records has become a bigger, more politically dangerous one: Whether the then-secretary of state and her close aides, in choosing to use a private email system, disregarded common sense and may have put sensitive information at risk of falling into the wrong hands.

The first warning came July 31 when a Justice Department prosecutor called Clinton’s longtime lawyer, David Kendall, seeking a thumb drive that contained a copy of the 30,000 emails that Clinton had turned over earlier to the State Department, according to a person briefed about the conversation.

Six days later, Kendall turned over to the FBI his thumb drive – and two copies – which had been stored in a safe in his office provided by the State Department. Next, the FBI wanted Clinton’s private server. That too, has been handed over, arriving in government hands Wednesday afternoon from the New Jersey data center where it was being kept.

The FBI’s interest in Clinton’s email system arose after the intelligence community’s inspector general referred the issue to the Justice Department on July 6. Intelligence officials have expressed concern that some sensitive information was not in the government’s possession and that Clinton’s unusual email system could have “compromised” secrets.

That investigation, still preliminary, is focusing on how to contain any damage from classified information that might have been put at risk. Officials have said that Clinton is not a target. But, according to legal experts, this type of security review can turn into a criminal investigation if there is evidence that someone intentionally mishandled government secrets.

Backers reject claims

Clinton’s Democratic backers in Congress, including some who have had access to the emails in question, have in recent days waved away the suggestion that Clinton bears responsibility for improper handling of sensitive information. Rep. Adam Schiff and Sen. Dianne Feinstein, two California Democrats who are ranking intelligence committee members, said none of the emails alleged to contain classified information was written by Clinton.

“I think the fact that some staff of the Department of State may have sent the secretary some emails not marked as classified is going to prove to be of very minor significance,” Schiff said.

The issues around Clinton’s emails also have intensified as it has become clear that a number of her statements defending her actions now appear to be false.

As she did that Sunday in Iowa, Clinton has said multiple times that she never sent or received any emails containing information that was classified at the time.

But the intelligence community’s inspector general, reviewing a small sample of Clinton’s private emails, has contradicted that claim. While the emails may not have been marked that way, they contained classified information. The IG said this week that he had discovered information in two emails that intelligence agencies considered to be top secret, the highest category of classification.

In recent weeks, Clinton, her campaign and the State Department have shifted their language, offering less definitive denials regarding classified information.

When her use of a private email account first came to light in March, Clinton said that, “I’m certainly well-aware of the classification requirements and did not send classified material.”

But then last week, as The Washington Post reported that the FBI had begun its inquiries, Clinton spokesman Nick Merrill said that she had not sent or received any emails that were “marked classified at the time.”

This week, when the inspector general said some emails should have been designated top secret, the State Department said it was reviewing the content but acknowledged that some had been sent over “unclassified systems” in 2009 and 2011 and “ultimately some were forwarded to Secretary Clinton.”

Like Merrill, the agency spokesman also said that the emails had not been marked as classified. According to federal rules, it is the responsibility of the sender of an email to ensure it contains the proper classification designations.

private accounts vulnerable

The controversy highlights one of the problems with Clinton’s decision to use her own email system during her four years running the State Department.

At many agencies that handle sensitive information, discussions of classified material and sharing of classified documents must take place on specially secure classified computers. The government protections don’t extend to private accounts.

Other Cabinet secretaries have been known to use private accounts, including one of Clinton’s predecessors, Colin Powell.

State Department rules require employees to “use, hold, process, or store classified material in data and word processing systems … only under conditions that will prevent unauthorized persons from gaining access,” according to the agency’s Foreign Affairs Manual.

A 2009 executive order also specifies that classified information “may not be removed from official premises without proper authorization.”

In several of the emails that intelligence officials now say are classified, Clinton’s closest aides were writing to alert her to specific information and sometimes citing what the CIA was reporting, according to two government officials who have reviewed them.

Clinton was on the thread of the emails as other aides shared new information or joined in the discussion, and often did not reply, the officials said.

John Fitzpatrick, head of the Information Security Oversight Office within the National Archives, said agencies train officials with security clearances to spot sensitive material and then to look up the proper classifications – such as “confidential,” “secret” or “top secret.”

“If you write an email , you are expected to distinguish the classified from the unclassified,” Fitzpatrick said. “If you say ‘the CIA reports’ something – writing that sentence should set off alarm bells.”

Fitzpatrick said the dispute is somewhat academic given Clinton’s unique system. Officials shouldn’t rely on private systems for work emails, he said, and they should never talk about classified matters on a private system.

One open question is whether 31,000 emails that Clinton deemed to be personal and which were not handed over to the government remain in some way on her server.

Both Clinton and a lawyer for the company that has been handling the server since 2013 have said there is no trace of any files on the drive.

But technology experts say that there can be ways to revive deleted emails.

Joseph Lorenzo Hall, the chief technologist for the Center for Democracy and Technology, said some ways of deleting information from a server are more thorough than others.

A quick deletion may erase data from the table of contents in a server, but the data itself could remain deep in the system.