US authorities probing the recent leak of secret materials are focusing on past CIA contractors who may have been upset over job losses, the Wall Street Journal has reported.

WikiLeaks delivered a blow to America's top spies by publishing earlier this month nearly 9,000 classified documents the anti-secrecy group said were part of the Central Intelligence Agency's entire arsenal of cyber-attack plans.

The WikiLeaks dump has set off an intense probe into how the materials, which detailed how the CIA focuses on breaking into personal electronics like smartphones, got away from the agency.

WikiLeaks had said earlier they got access to the trove via a circle of private contractors, and the group's founder Julian Assange chided the spy agency as careless to have lost the materials.

Investigators initially zeroed in on a small team of software developers who reportedly had high-level security clearances and worked with the CIA on hacking projects, according to the daily.

Citing unnamed sources, the paper said that "bad blood" among contractors working for the US spy agency may have left some workers "disgruntled," particularly after job cuts.

A leak from contractors would not be a complete surprise, as several other releases in recent years have been attributed to experts hired by the intelligence community.

The CIA's sister spy body, the National Security Agency, was rocked in 2013 when contractor Edward Snowden released documents showing how it secretly raked up data from Americans' telecommunications and spied on US allies.

Early last year, a secretive hacking group called the Shadow Brokers offered for sale online a batch of hacking tools stolen from the NSA.

And in late 2016, the NSA discovered that another contractor, Harold Martin, had removed to his home an estimated 50 terabytes worth of data and documents, including reportedly sensitive hacking tools.

Major leaks have not all come from contractors, however.

Chelsea Manning, whose leak of hundreds of thousands of pages of diplomatic communications in 2010 made WikiLeaks famous, was a US army intelligence analyst at the time.