(CNN) -- Hackers on Friday targeted two Brazilian government sites, the latest in a string of attacks by at least two hacker outfits.
The website for Brazil's Institute of Geography and Statistics, or IBGE, which oversees the census, was temporarily replaced by a message from a group calling itself "Fail Shell."
A screenshot of the hacked web page shows the image of an eye with a Brazilian flag imposed on it.
"This month, the government will experience the greatest number of virtual attacks in its history by Fail Shell. These attacks are a form of protest of a nationalist group that wants to make Brazil a better country," the message said in part.
The IBGE site was subsequently taken offline.
Later in the day, the country's ministry of sports suffered a denial-of-service, or DoS, attack, CNN affiliate Globo TV reported. A DoS attack involves saturating the targeted website with so much fake traffic that it overloads or cannot respond to legitimate visitors. It was unknown who carried out this attack.
This week alone, hackers have attacked the websites for Brazilian President Dilma Rousseff, the country's tax collection agency and the state-run oil company Petrobras.
According to the ministry of sports, the attack was superficial, the system was not altered and no data was stolen, the state-run Agencia Brasil news agency reported.
The presidential website and tax website also suffered denial-of-service attacks, Agencia Brasil reported. According to the government, the attacks were carried out with nearly 2 billion hits on each page.
Another hacker group, LulzSecBrazil, claimed responsibility on its Twitter page for the attacks on the government sites. Although the government said that no data was stolen, LulzSecBrazil posted images of what appeared to be internal documents for some of the government agencies.
"We want to put our feet on the street and claim our rights as citizens! Embark with us on this mission," says a message on the group's Twitter account. The group claims to be against vandalism and for standing up for justice of the people.
The two hacker groups, LulzSecBrazil and Fail Shell, do not appear to be working together. In its message, Fail Shell says there is "no room" in Brazil for hackers without ideology, lumping LulzSecBrazil into that category.