At least four emails from the private email account that former US Secretary of State Hillary Clinton used while in office contained classified information, according to a government inspector's letter that deepened the email controversy dogging Clinton's 2016 presidential campaign.
The inspector general of the intelligence community wrote in a letter to members of Congress on Thursday that a sampling of 40 of 30,000 Clinton emails found at least four that should have been classified as secret.
The information in the emails was derived from the US intelligence community "and should have been handled as classified, appropriately marked and transmitted via a secure network," the inspector general, Charles McCullough, wrote. His letter was made available on Friday on a House of Representatives website.
Clinton's use of her private email account for her work as America's top diplomat came to light in March and drew fire from political opponents who accused her of sidestepping transparency and record-keeping laws.
The former first lady has said she sent no classified information on her private email account. The inspector general's comments left open the possibility that Clinton sent or received emails with classified material that she was not aware of.
The front-runner to represent the Democratic Party in the November 2016 election, Clinton has repeatedly said she broke no laws or rules by eschewing a standard government email account.
Clinton spokesman Nick Merrill said in a statement she "followed appropriate practices in dealing with classified materials."
Clinton handed over some 30,000 emails from the private account to the State Department after she quit in 2013, but many thousands of others that she says are not related to her work were deleted.
"We are all accountable to the American people to get the facts right, and I will do my part," Clinton said at a speech in New York, noting she had handed over tens of thousands of pages of emails.
McCullough said State Department officials had told his office "that there are potentially hundreds of classified emails within the approximately 30,000 provided by former Secretary Clinton."
Republicans in Congress and on the presidential campaign trail have seized on the email scandal to portray Clinton as continuing secretive practices they say they also characterized President Bill Clinton's eight years in office.
While Clinton faces little competition for the Democratic Party's nomination, several recent polls have found a majority of voters find her untrustworthy, a perception exacerbated by controversy over her emails.
The US Justice Department said on Friday it is weighing a request by two government inspectors to look into the possible mishandling of classified information from Clinton's private email account.
The department said earlier in the day it had been asked to treat the issue as a potential criminal matter, but later backtracked to drop a reference to any criminal investigation.