A $10 billion-a-year effort to protect sensitive government data, from military secrets to Social Security numbers, is struggling to keep pace with an increasing number of cyberattacks and is being undermined by unwitting federal employees and contractors.

Workers scattered across more than a dozen agencies, from the Defense and Education departments to the National Weather Service, are responsible for at least half of the federal cyberincidents reported each year since 2010, according to an Associated Press analysis of records.

They have clicked links in bogus phishing emails, opened malware-laden websites and been tricked by scammers into sharing information.

One was redirected to a hostile site after connecting to a video of tennis star Serena Williams. A few act intentionally, most famously former National Security Agency contractor Edward Snowden, who downloaded and leaked documents revealing the government’s collection of phone and email records.

At a time when intelligence officials say cybersecurity trumps terrorism as the No. 1 threat to the United States — and when breaches at businesses such as Home Depot and Target focus attention on data security — the federal government isn’t required to publicize its own data losses.

Last month a breach of unclassified White House computers by hackers thought to be working for Russia was reported not by officials, but by the Washington Post. Congressional Republicans complained that even they weren’t alerted to the hack.

To determine the extent of federal cyberincidents, the AP filed dozens of Freedom of Information Act requests; interviewed hackers, cybersecurity experts and government officials; and obtained documents describing digital cracks in the system.

That review shows that 40 years and more than $100 billion after the first federal data protection law was enacted, the government is struggling to close holes without the knowledge, staff or systems to outwit an ever-evolving foe.

Fears about breaches have been around since the late 1960s, when the federal government began shifting its operations onto computers. Officials responded with software designed to sniff out malicious programs and raise alarms about intruders. And yet attackers have always found a way in, exposing tens of millions of sensitive and private records that include employee usernames and passwords and veterans’ medical files.

From 2009 to 2013 the number of reported breaches just on federal computer networks — those with addresses ending in .gov or .mil — rose to 46,605 from 26,942, according to the U.S. Computer Emergency Readiness Team. Last year US-CERT responded to a total of 228,700 cyberincidents involving federal agencies, companies that run critical infrastructure, and contract partners. That’s more than double the incidents in 2009.

And employees are to blame for at least half of the problems.

Last year, for example, about 21 percent of all federal breaches were traced to government workers who violated policies; 16 percent whose devices were lost or stolen; 12 percent who improperly handled sensitive information printed from computers; at least 8 percent who ran or installed malicious software; and 6 percent who were enticed to share private information, according to an annual White House review.

Documents released to the AP show how workers were lured in.

In one incident during Christmastime 2011, Education Department employees received an email purportedly from Amazon.com that asked them to click on a link. Officials quickly warned staff that it could be malicious. The department did not release information to the AP about any resulting damage.

Reports from the Defense Department’s Defense Security Service, tasked with protecting classified information and technologies in the hands of federal contractors, show how easy it is for hackers to get into DOD networks. One military user received messages that his computer was infected when he visited a website about schools. Officials tracked the attacker to what appeared to be a German-based server.

"We’ll always be vulnerable to … human-factor attacks unless we educate the overall workforce," said Assistant Secretary of Defense Eric Rosenbach, a cybersecurity adviser.

Although the government is projected to spend $65 billion on cybersecurity contracts between 2015 and 2020, many experts say the effort is not enough to counter a growing pool of hackers whose motives vary. Russia, Iran and China have been named as suspects in some attacks, while other valuable data are sought by thieves. Only a small fraction of attackers are caught.

For every thief or hostile state, there are tens of thousands of victims like Robert Curtis, who was besieged by identity thieves after someone stole data tapes left in a car in a Texas garage in 2011.

Curtis declined to talk about specifics of his case, but according to court records, the thief took unencrypted computer tapes containing health records of about 5 million current and former Pentagon employees and their families. The car belonged to an employee of a federal contractor tasked with securing those records.

"I was angry because we as citizens trust the government to act on our behalf," Curtis said.

Criminals have tried to get cash, loans, credit — even establish businesses — in Curtis’ name, according to court records. He and his wife have frozen bank and credit accounts.

"It is very ironic," said Curtis, himself a cybersecurity expert who worked to secure networks at the Pentagon. "I was the person who had paper shredders in my house. I was a consummate data protection guy."

Martha Mendoza, Associated Press