WASHINGTON • The US authorities investigating the recent leak of secret materials are focusing on past CIA contractors who may have been upset over job losses, according to the Wall Street Journal (WSJ).

WikiLeaks delivered a blow to America's top spies by publishing earlier this month nearly 9,000 classified documents the group said were part of the Central Intelligence Agency's (CIA) 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 said earlier it got access to the trove via private contractors. Its founder Julian Assange said the CIA was careless.

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 WSJ on Sunday.

Citing unnamed sources, the paper said "bad blood" may have left some "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 (NSA), 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 hacking group offered for sale a batch of hacking tools stolen from the NSA.

Later that year, the NSA discovered that another contractortook home an estimated 50TB worth of data and documents.

AGENCE FRANCE-PRESSE