SAN FRANCISCO — Want to invisibly spy on 10 iPhone owners without their knowledge? Gather their every keystroke, sound, message and location? That will cost you $650,000, plus a $500,000 setup fee with an Israeli outfit called the NSO Group. You can spy on more people if you would like — just check out the company’s price list.

The NSO Group is one of a number of companies that sell surveillance tools that can capture all the activity on a smartphone, like a user’s location and personal contacts. These tools can even turn the phone into a secret recording device.

Since its founding six years ago, the NSO Group has kept a low profile. But last month, security researchers caught its spyware trying to gain access to the iPhone of a human rights activist in the United Arab Emirates. They also discovered a second target, a Mexican journalist who wrote about corruption in the Mexican government.

Now, internal NSO Group emails, contracts and commercial proposals obtained by The New York Times offer insight into how companies in this secretive digital surveillance industry operate. The emails and documents were provided by two people who have had dealings with the NSO Group but would not be named for fear of reprisals.