WASHINGTON (Bloomberg)—U.S. Attorney General Eric Holder said the Justice Department is examining cyber attacks that have been blamed on WikiLeaks supporters.

“We are aware of the incidents,” Mr. Holder said in Washington today. “We are looking into them.”

Homeland Security Secretary Janet Napolitano told the same news conference the United States is coordinating with the “private sector” on cyber issues. Separately, a Department of Homeland Security official said the agency is working with companies to limit damages from the attacks.

MasterCard Inc.’s website was slowed yesterday by a flood of traffic in a denial-of-service attack after the company blocked use of its network by WikiLeaks. Visa Inc. took steps yesterday to restore its website to full operation after experiencing heavier than normal traffic.

Computer-security analysts said prosecutions of those behind the attack aren’t clear cut.

“At an individual level a person is pushing a button and sending a packet,” said Robert Gourley, a former cyber-security expert with the Defense Intelligence Agency. “I don’t know what legal precedents there would be that allow you to take a person to court for doing this.”

MasterCard, the world’s second-biggest payments processor, and London-based Visa Europe Ltd. said on Dec. 7 they were suspending use of their networks by WikiLeaks after the organization on Nov. 28 began publishing secret U.S. military and State Department documents online.

Amazon.com Inc. and EBay Inc.’s PayPal unit also recently cut off access to WikiLeaks. PayPal said it was taking additional steps today as it sustained cyber attacks.

In an e-mailed statement, PayPal said it has temporarily strengthened the vetting process for new developers who want to build on PayPal’s software code.

“We have taken this temporary action to focus on maintaining business as usual for our millions of active PayPal customers, merchants and developers,” the company said.

The hackers turned to PayPal after failing to take down Amazon’s site, according to a post on Twitter that claims to represent the hackers.

“We have changed our target—the Hive isn’t big enough to attack Amazon,” the post said. “New target: api.paypal.com.”

Craig Berman, a spokesman for Amazon, declined to comment.

The impact of the attacks probably won’t affect the operations of big Internet companies, said Scott Kessler, an equity analyst at Standard & Poor’s in New York.

“If those were to succeed, they typically don’t last very long,” Kessler said. “You will see them taking hold for a few hours at most. These companies are, generally speaking, well prepared to handle these types of attacks.”

Security consultants said one likely result of the attacks is that federal authorities will seek greater cooperation from the country’s largest Internet providers, which are in the best position to shut the attacks down quickly.

“These companies have typically been reticent,” said Anup Ghosh, a former senior scientist at the Defense Advanced Research Projects Agency. Still, “they are really the organizations that can do something about it.”

Earlier today, Swedish law enforcement officials said they are probing cyber attacks that crashed government websites following the arrest in the U.K. of WikiLeaks founder Julian Assange, who is fighting extradition to Sweden.

The Swedish Prosecution Authority said its website was “deliberately attacked” on the night of Dec. 7 and referred the matter for investigation. Mr. Assange, a 39-year-old Australian, is in a London jail after Sweden issued an international arrest warrant over rape and molestation allegations.