LONDON – Computer attacks ricocheted through cyberspace Thursday between WikiLeaks supporters and the companies they accuse of trying to stifle the group, with websites on both sides taken out of service from time to time.

Hackers launched attacks on MasterCard, Visa, Swedish prosecutors, a Swiss bank and others who have acted against the WikiLeaks site and its jailed founder Julian Assange.

Sites allied with WikiLeaks continued to be attacked but remained active.

The Justice Department is looking into the cyber attacks on opponents of WikiLeaks and companies that have stopped doing business with it, Attorney General Eric Holder said Thursday.

Holder made the comment in Washington at a news conference following a meeting with European Union law enforcement partners on cybersecurity, counterterrorism and data protection.

In addition, U.S. officials say WikiLeaks’ publication of secret diplomatic documents have thrown diplomacy into disarray, caused countries to curtail dealings with America and, in the case of an earlier release of classified military documents, put the lives of informants at risk.

While U.S. allies have also criticized WikiLeaks, some world leaders have questioned the arrest of Assange.

Russian Prime Minister Vladimir Putin, questioning the reliability of leaked U.S. cables referring to his nation as undemocratic and corrupt, said the fact that Assange is in custody shows the West has its own problems with democracy.

“Why was Mr. Assange hidden in prison?” Putin asked at a news conference. “Is this democracy?”

Brazilian President Luiz Inacio Lula da Silva said he was surprised by the lack of outcry against Assange’s arrest.

“This WikiLeaks guy was arrested and I’m not seeing any protest for freedom of expression,” Silva said Thursday in Brasilia. “There is nothing, nothing for freedom of expression and against the imprisonment of this guy who was doing better work than many of the ambassadors.”

Many U.S.-based Internet companies have cut their ties to WikiLeaks, including MasterCard Inc., Visa Inc., Amazon.com, PayPal Inc. and EveryDNS. Those moves have hurt WikiLeaks’ ability to accept donations and support publishing efforts — and touched off a bout of Web-based warfare.

Retaliatory attacks — which WikiLeaks says it does not sanction — have been claimed by a loose-knit group of “hacktivists” who gather under the handle “Anonymous.”

They are using a modified version of software generally used to conduct “stress testing” on websites, according to Paul Mutton, an analyst with the London-based company Netcraft, which is tracking the attacks.

Even unsophisticated supporters can participate in attacks — all they have to do is download the file, which is then remotely operated to send a stream of bogus page requests to target websites.