There are "no signs or indications" that Snowden had accomplices or tried to sell secrets, a U.S. official said. Investigators think the leaker is still in Hong Kong and have a general sense of where he is in that Asian metropolis.
Snowden's case has become a hot issue in that coastal city, making local newspaper front pages, stirring legal debates and prompting plans for a rally in support of him over the weekend.
The reaction in mainland China, on the other hand, has been muted. State-run media outlets have covered the case cautiously, appearing to try to avoid focusing too much attention on some of the sensitive issues his disclosures have raised, such as government surveillance of citizens.
The Snowden story has also so far failed to make big waves among China's tens of millions of highly active social media users.
Some Chinese state media have taken the opportunity to highlight Snowden's comments to the South China Morning Post alleging that the U.S. government has hacked Chinese targets.
In recent years, the Global Times newspaper said in an editorial, "the United States has always claimed itself to be a victim of Chinese hacking activities. Many speculate that it's a coverup for hacking activities conducted by the U.S. government. Now, Snowden's revelation proves that such activities have already been going on for a long time."
Among some 61,000 reported targets of the National Security Agency, Snowden told the Hong Kong newspaper, are hundreds of computers in China.
U.S. officials have increasingly accused China of being the source of thousands of attacks on U.S. military and commercial networks. Beijing has denied such attacks.
The South China Morning Post said it had seen documents provided by Snowden but was unable to verify their authenticity. The newspaper also said it was unable to independently verify allegations of U.S. hacking of networks in Hong Kong and mainland China since 2009.
Snowden told the paper that some of the targets included the Chinese University of Hong Kong, public officials and students. The documents also "point to hacking activity by the NSA against mainland targets," it reported.
The claims came just days after Obama pressed Chinese President Xi Jinping to address cyberattacks emanating from China that Obama described as "direct theft of United States property."