WASHINGTON (AP) -- Hillary Rodham Clinton's private email server, containing an electronic inventory of some 55,000 pages of emails from her stint as secretary of state, was repeatedly hit by attempted cyberattacks originating in China, South Korea and Germany in 2014, according to a congressional document obtained by The Associated Press.

At least five cyberattack tries were apparently blocked by a "threat monitoring" product that was connected to her network in October 2013, eight months after she left office. But for more than three months earlier that year, her system was not protected by the security product because it had not been installed.

Clinton's server operated without its threat monitoring protection between June and October 2013, according to a letter sent by Sen. Ron Johnson, R-Wis., chairman of the Homeland Security and Government Affairs Committee. That means her server was possibly vulnerable to cyberattacks during that time.

It was not immediately clear whether the attempted intrusions into Clinton's server were serious espionage threats or the sort of nuisance attacks that hit computer servers the world over. But the revelations underscore the extent to which any private email server is a target, raising further questions about Clinton's decision to undertake sensitive government business over private email stored on a homemade system.

Johnson's letter to Victor Nappe, CEO of SECNAP, the company that provided the threat monitoring product, seeks a host of documents relating to the company's work on Clinton's server and the nature of the cyber intrusions detected. Johnson's committee is investigating Clinton's email arrangement.

Clinton has not said what, if any, firewall or threat protection was used on her email server before June 2013, including the time she was secretary of state from 2009 to 2013 and the server was kept in her home in the New York City suburbs.

A February 2014 email from SECNAP reported that malicious software based in China "was found running an attack against" Clinton's server. In total, Senate investigators have found records describing three such attempts linked to China, one based in Germany and one originating in South Korea. The attacks occurred in 2013 and 2014. The letter describes four attacks, but investigators have since found records about a fifth attempt, said officials who were not authorized to discuss the matter publicly.

Any hackers who got access to her server in 2013 or 2014 could have stolen a trove of sensitive email traffic involving the foreign relations of the United States. Thousands of Clinton emails made public under the Freedom of Information Act have been heavily redacted for national security and other reasons.

Clinton "essentially circumvented millions of dollars' worth of cybersecurity investment that the federal government puts within the State Department," said Justin Harvey, chief security officer of Fidelis Cybersecurity.

"She wouldn't have had the infrastructure to detect or respond to cyber attacks from a nation-state," he said. "Those attacks are incredibly sophisticated, and very hard to detect and contain. And if you have a private server, it's very likely that you would be compromised."

A spokesman for the Clinton campaign did not answer detailed questions from The Associated Press about the cyber intrusions. Instead, spokesman Brian Fallon attacked Johnson by linking him to the House Benghazi committee inquiry, which the campaign dismissed in a recent media ad as politically motivated.