German media reports say users and supporters of a popular online anonymity tool are among those automatically singled out for special attention by US security services.
The report by public broadcasters WDR and NDR says the code from the National Security Agency's XKeyscore software reveals the NSA's interest in anyone who uses a program called Tor that can obscure a person's digital trail.
The code for the software, which was created to filter through vast amounts of data to find information of interest to US intelligence, also monitors a handful of key computers that act as phone books for the Tor network, the report said.
One of those computers reportedly belongs to Sebastian Hahn, a German student and online privacy activist.
The report came on the day German lawmakers began hearing expert testimony for a probe into the activities of foreign intelligence agencies in Germany. The inquiry was sparked by reports based on documents leaked by former NSA contractor Edward Snowden, which showed that German citizens, including Chancellor Angela Merkel, were targeted by US intelligence.
Former US intelligence official William Binney told the lawmakers that Thursday's report was plausible.
Binney, who left the NSA in 2001 alleging surveillance overreach after the 9/11 terror attacks, said factors that put people into "zones of suspicion" include visiting certain websites regularly.
The report published by WDR and NDR was co-authored by three privacy activists linked to the developers of Tor. One of them, Jacob Appelbaum, was previously involved in the publication of reports based on Snowden documents.
The creation of Tor was partly funded by the US government to help dissidents in authoritarian countries communicate freely. But law enforcement agencies around the world have claimed that it also makes the work of identifying online criminals harder.