A free tool released Thursday allows users to scan their computers for surveillance malware that has been used to spy on journalists and activists.

The open source tool Detekt is being released in partnership with human rights group Amnesty International, German digital rights group Digitale Gesellschaft, the Electronic Frontier Foundation and Privacy International.

“Our ultimate aim is for human rights defenders, journalists and civil society groups to be able to carry out their legitimate work without fear of surveillance, harassment, intimidation, arrest or torture,” Amnesty International said in an online posting introducing Detekt.

Whistleblower Edward Snowden exposed the extent of government surveillance on activists and citizens. Amnesty said it is concerned about a chill on human rights activists and journalists, especially those in repressive countries, because of such surveillance.

Detekt scans Windows-based computers for common surveillance tools used in spying on activists. Those who detect a threat on their computer can then take steps to remove it.

The software developers warn that there is no guarantee that Detekt will find every bit of spyware, as new technology is continually being developed.

“Beware that it is possible that Detekt may not successfully detect the most recent versions of those malware families,” the developers wrote on the site resistsurveillance.org, which introduces Detekt.

“Indeed, some of them will likely be updated in response to this release in order to remove or change the patterns that we identified. In addition, there may be existing versions of malware, from these families or from other providers, which are not detected by this tool. If Detekt does not find anything, this unfortunately cannot be considered a clean bill of health.”

Commercial entities have developed and sold surveillance tools that read emails and instant messaging conversations, listen in on Skype calls and remotely control a computer’s camera and microphone and sold them around the world.

There is no regulation against these technologies being used by repressive governments, Amnesty says, estimating the market for surveillance software at $5 billion US a year.

“European and American companies have been quietly selling surveillance equipment and software to countries across the world that persistently commit serious human rights violations,” it said.

Amnesty is urging governments to take action to stop the spread of spyware and calls on industry to consider the human rights records of countries where it sells such software.

Targeted surveillance is only legitimate when it is based on suspicion of criminal activity and backed up by a legal order, the group said.