Members of the Anonymous hacking movement have released hundreds of megabytes of documents that they say were stolen from government security contractor ManTech.

The files released on the Pirate Bay filesharing site late Friday contain nearly 400 megabytes of documents, spreadsheets, résumés, planning documents, even photographs, that appear to have come from the government contractor.

A ManTech representative declined to comment on the incident Friday, but in a note posted to the company's website, ManTech did not dispute the claim that it had been hacked.

"All organisations attract cyber threats in our highly networked world," the company said. "Our practice is generally not to comment on reports involving security related matters. However, given current publicity, we wish to assure our customers, employees, shareholders and business partners that ManTech takes seriously recent reports of a cyber threat, and we responsibly and actively address all sources of information about threats to our information and assets and those of our customers."

Anonymous said it was releasing the data to embarrass the government contractor, which recently signed a five year deal to provide managed security services for the FBI.

"It also remains to be seen how much longer the public will accept how completely incompetent law enforcement agencies are spending their citizens' money to fund even more incompetent federal contractors," Anonymous said. "It's really good to know that you guys are taking care of protecting the Unites States from so-called cyber threats."

Anonymous is currently at war with the FBI, which has been investigating scores of its members. Earlier this month, the FBI arrested 14 people in connection with a December 2010 Anonymous attack on PayPal's website, and it has conducted many more raids in response to a series of distributed denial-of-service attacks coordinated by Anonymous on websites such as PayPal, MasterCard and Visa, after they stopped processing donations to Wikileaks.

An FBI spokeswoman declined to comment on the alleged ManTech hack.

A contractor to intelligence agencies, the Departments of State, Defense and Homeland Security, ManTech has more than 10,000 employees worldwide. In 2010 it reported $2.9 billion (£1.76 billion) in revenue.

But the company is just the latest security contractor to be hit by Anonymous and its sister organisation, LulzSec. Contractors HBGeary and Unveillance were also embarrassed by high profile breaches this year, and LulzSec has targeted the CIA, the US Senate and members of the FBI's InfraGuard information-sharing programme, among others.