(CNN) -- The private intelligence firm Stratfor called the release of some of its e-mails by WikiLeaks a "deplorable, unfortunate -- and illegal -- breach of privacy."
"In December, thieves compromised Stratfor's data systems and stole a large number of company e-mails, along with other private information of Stratfor readers, subscribers and employees," it said Sunday in a Facebook posting. "Those stolen e-mails apparently will be published by WikiLeaks."
The Austin, Texas-based firm added, "Some of the e-mails may be forged or altered to include inaccuracies; some may be authentic. We will not validate either. Nor will we explain the thinking that went into them."
In a statement, WikiLeaks said it had begun publishing the e-mails, which are dated between July 2004 and December and exceed 5 million in number. "They reveal the inner workings of a company that fronts as an intelligence publisher, but provides confidential intelligence services to large corporations ... and government agencies," the statement says. "The e-mails show Stratfor's web of informers, pay-off structure, payment laundering techniques and psychological methods."
WikiLeaks, a website that facilitates the leaking of confidential information, says the documents will be released in coming weeks through a network of more than 25 news outlets and activist groups.
The first document out was titled "The Stratfor Glossary of Useful, Baffling and Strange Intelligence Terms," featuring brief and sometimes humorous definitions and blunt assessments of U.S. intelligence and law enforcement.
Others focused on speculation about the health of Venezuelan President Hugo Chavez and who may have been behind a suspected campaign of sabotage against Iran's nuclear program.
An FBI spokeswoman had no comment.
WikiLeaks has published hundreds of thousands of U.S. military and State Department documents, including field reports from the Iraq and Afghanistan wars and embassy cables that feature the candid assessments of U.S. diplomats. A U.S. soldier, Pfc. Bradley Manning, faces a court-martial on charges that he leaked the documents to the website.
WikiLeaks founder Julian Assange, meanwhile, is in Britain battling an extradition request from prosecutors in Sweden who want to question him about unrelated accusations of sexual assault. Assange has not been charged with a crime and denies wrongdoing.
Stratfor has been targeted by hackers who have released private data about subscribers in recent months, prompting the company to offer its clients a year of paid identity-protection coverage.
The firm said in a statement Monday that thieves compromised its data systems in December and stole a number of company e-mails, along with private data about its subscribers, employees and readers.
"Having had our property stolen, we will not be victimized twice by submitting to questioning about them," the company said about the leaked e-mails.
It described the release as "another attempt to silence and intimidate the company, and one we reject."
"Stratfor will not be silenced and will continue to publish the geopolitical analysis our friends and subscribers have come to rely upon," the statement said.
Describing the e-mails as private property, Stratfor said they were written casually, with no expectation that anyone outside the communication chain would see them.
"They should be read as such," the company said. "Stratfor understands that this hack and the fallout from it, including the disclosures by WikiLeaks, have created serious difficulties for our subscribers, friends and employees."
CNN's Carol Cratty contributed to this report.