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In recent years, OSINT has come to encompass an array of unclassified material available online or for purchase. That includes social media and posts on web forums as well as commercially available data collected by technology giants, advertising entities and consumer-facing companies. It also includes a huge category of data that modern electronic devices emit and that are often collected without much consumer awareness, such as precise geolocation, IP addresses, wireless identifiers associated with cars and modern Bluetooth devices.

The government has also invested heavily in developing specialized analytical capabilities to extract information from open sources--for example, tools that can estimate location information from social-media photos using tiny clues such as the position of the sun, stars or visible landmarks like mountains.

Depending on the intelligence priority, these data sets can now be equally or more valuable to government agencies than more traditional, intrusive types of government surveillance like wiretaps, some former U.S. intelligence officials have said. Unlike such sources, open-source retrieval doesn't require approval from the Foreign Intelligence Surveillance Court.

Open-source intelligence has played a significant role in military operations in recent years, especially in the global campaigns against al Qaeda and Islamic State, according to people familiar with the matter and documents viewed by The Wall Street Journal. Some of the lethal airstrike targeting in the campaign against Islamic State was done in part through social media--including by identifying and killing terrorist targets who made critical operational security errors like leaving GPS coordinates attached to public social-media posts, the people say.

Foreign governments have come to rely on open-source intelligence, too. The pervasiveness of what some in the field call "digital dust" in modern society has complicated efforts of U.S. spies to remain undercover in overseas postings, according to current and former officials.

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