Anonymous claims its hacked the US Census Bureau. Flickr/Anonymous9000 A group of hackers claims it has successfully hacked the US Census Bureau, compromising over 4,200 workers’ data in the process.

The hackers claim to be part of the loose-knit Anonymous hacktivist collective. Anonymous is a collective brand used by fragmented groups of hackers. The hackers have not revealed any specific details about how they breached the Census Bureau, but have published data alleging to be stolen during the raid online.

The data was allegedly taken from the Census Bureau’s databases, and includes the usernames, work email addresses, and office phone numbers of the government department’s staff.

A Census Bureau spokesperson confirmed to Business Insider an attack did occur, but added no internal systems were affected.

"The US Census Bureau is investigating an IT security incident relating to unauthorized access to non-confidential information on an external system that is not part of the Census Bureau internal network. Access to the external system has been restricted while our IT forensics team investigates," said the spokesperson

"Security and data stewardship are integral to the Census Bureau mission. We will remain vigilant in continuing to take every necessary precaution to protect all information."

Why Anonymous did it

The hackers claim they mounted the attacks to protest the Transatlantic Trade and Investment Partnership (TTIP) and Trans-Pacific Partnership (TPP) trade negotiations.

The trade negotiations are designed to improve trade relations and lower barriers between participating nations.

Specifically, they work to establish a common framework for intellectual property, enforce standards for labour law and environmental law, and establish an investor-state dispute settlement mechanism.

The talks have also explored opening Europe’s public health, education and water services to US companies. This has led to concerns within Europe that free, state-run health services, such as the UK's NHS, could be privatised.

Yet another headache for the US government’s IT team

The Census Bureau breach follows a larger attack on the US Office of Personnel Management (OPM).

The White House revealed the attack in June when it admitted hackers had breached the Office of Personnel Management (OPM), compromising the details of roughly four million staff.

The attack was initially believed to be the work of state-sponsored Chinese hackers. However, US President Barack Obama’s administration said it will not officially blame China for the OPM hack, or take action against it, in the immediate future on July 22.